Add editor contribution guides.

engine/editor/creating_icons.rst

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+.. _doc_editor_icons:
+
+Editor icons
+============
+
+When a new class is created and exposed to scripting, the editor's interface
+will display it with a default icon representing the base class it inherits
+from. In most cases, it's still recommended to create icons for new classes to
+improve the user experience.
+
+Creating icons
+~~~~~~~~~~~~~~
+
+To create new icons, you first need a vector graphics editor installed.
+For instance, you can use the open source `Inkscape <https://inkscape.org/>`_ editor.
+
+Clone the ``godot`` repository containing all the editor icons:
+
+.. code-block:: bash
+
+    git clone https://github.com/godotengine/godot.git
+
+The icons must be created in a vector graphics editor in SVG format. There are
+two main requirements to follow:
+
+- Icons must be 16×16. In Inkscape, you can configure the document size in
+  **File > Document Properties**.
+- Lines should be snapped to pixels whenever possible to remain crisp at lower DPI.
+  You can create a 16×16 grid in Inkscape to make this easier.
+
+Once you're satisfied with the icon's design, save the icon in the cloned
+repository's ``editor/icons`` folder. The icon name should match the intended
+name in a case-sensitive manner. For example, to create an icon for
+CPUParticles2D, name the file ``CPUParticles2D.svg``.
+
+Color conversion for light editor themes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If the user has configured their editor to use a light theme, Godot will
+convert the icon's colors based on a
+`set of predefined color mappings <https://github.com/godotengine/godot/blob/master/editor/themes/editor_color_map.cpp>`__.
+This is to ensure the icon always displays with a sufficient contrast rate.
+Try to restrict your icon's color palette to colors found in the list above.
+Otherwise, your icon may become difficult to read on a light background.
+
+Icon optimization
+~~~~~~~~~~~~~~~~~
+
+Because the editor renders SVGs once at load time, they need to be small
+in size so they can be efficiently parsed. When the
+:ref:`pre-commit hook <doc_code_style_guidelines_pre_commit_hook>` runs, it automatically optimizes
+the SVG using `svgo <https://github.com/svg/svgo>`_.
+
+.. note::
+
+    While this optimization step won't impact the icon's quality noticeably, it
+    will still remove editor-only information such as guides. Therefore, it's
+    recommended to keep the source SVG around if you need to make further
+    changes.
+
+Integrating and sharing the icons
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you're contributing to the engine itself, you should make a pull request to
+add optimized icons to ``editor/icons`` in the main repository. Recompile the
+engine to make it pick up new icons for classes.
+
+It's also possible to create custom icons within a module. If you're creating
+your own module and don't plan to integrate it with Godot, you don't need to
+make a separate pull request for your icons to be available within the editor
+as they can be self-contained.
+
+For specific instructions on how to create module icons, refer to
+:ref:`Creating custom module icons<doc_custom_module_icons>`.
+
+Troubleshooting
+~~~~~~~~~~~~~~~
+
+If icons don't appear in the editor, make sure that:
+
+1. Each icon's filename matches the naming requirement as described previously.
+
+2. ``modules/svg`` is enabled (it should be enabled by default). Without it,
+   icons won't appear in the editor at all.
+
+References
+~~~~~~~~~~
+
+-  `editor/icons <https://github.com/godotengine/godot/tree/master/editor/icons>`__

engine/editor/editor_style_guide.rst

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+.. _doc_editor_style_guide:
+
+Editor style guide
+==================
+
+Introduction
+------------
+
+Thanks for your interest in contributing to the Godot editor!
+
+This page describes the grammar and writing style used throughout the Godot
+editor. Following this style guide will help your contribution get merged faster
+since there will be fewer review steps required.
+
+Writing style
+-------------
+
+- **Write messages (errors, warnings, ...) as full sentences.** They should start
+  with an uppercase letter and end with a period.
+- **Try to keep sentences short.** This makes it more likely that their translations
+  will be short as well, which is a good thing to avoid UI bugs.
+- **Use contractions.** For example, use "isn't" instead of "is not". An exception
+  to this rule can be made when you specifically want to emphasize one of the
+  contraction's words.
+- **Use double quotes in messages** (``""``) instead of single quotes (``''``).
+  Double quotes should be used to quote user input, file paths and possibly
+  other things depending on the context.
+
+.. seealso::
+
+    Try to follow the :ref:`doc_docs_writing_guidelines` in addition to the
+    guidelines outlined above.
+
+Button and menu texts
+---------------------
+
+Capitalize text in buttons and menu actions:
+
+- **Good:** *Open Editor Data Folder*
+- **Bad:** *Open editor data folder*
+
+If a menu action opens a modal dialog, suffix it with an ellipsis (``...``).
+
+- **Good:** *Editor Settings...*
+- **Bad:** *Editor Settings*
+
+Inspector sections
+------------------
+
+In general, don't create sections that contain less than 3 items. Sections that
+contain few items make it difficult to navigate the inspector, while missing the
+benefits of using sections such as folding.
+
+There are some valid exceptions for this, such as material features in
+:ref:`class_StandardMaterial3D`.
+
+This advice also applies to the Project Settings and Editor Settings.
+
+Inspector performance hints
+---------------------------
+
+Enum properties that noticeably impact performance should have a performance
+hint associated. The hint should refer to the *absolute* performance impact,
+rather than being relative to the other options provided in the enum. Here are
+some examples taken from the Godot editor:
+
+- **Screen-space antialiasing:** *Disabled (Fastest), FXAA (Fast)*
+- **MSAA quality:** *Disabled (Fastest), 2x (Fast), 4x (Average), 8x (Slow), 16x
+  (Slower)*
+- **DirectionalLight mode:** *Orthogonal (Fast), PSSM 2 Splits
+  (Average), PSSM 4 Splits (Slow)*
+
+For consistency, try to stick to the terms below (from fastest to slowest):
+
+- *Fastest, Faster, Fast, Average, Slow, Slower, Slowest*
+
+Their usage doesn't have to be contiguous. For example, you can use only "Fast"
+and "Slow" from the list above.
+
+If you're adding a new enum, its values should be ordered from the fastest
+option to the slowest option.
+
+Tooltips
+--------
+
+Consider adding tooltips whenever the action performed by a button or menu
+action isn't obvious. You can also provide additional context or highlight
+caveats in the tooltip.
+
+You can do this by calling ``set_tooltip(TTR("Text here."))`` on the
+Control-based node in question. If the tooltip is particularly long (more than
+~80 characters), wrap it over several lines by adding line breaks using ``\n``.
+
+Tooltips should follow the writing style described above. In addition to this,
+use indicative mood instead of imperative mood:
+
+- **Good:** *Computes global illumination for the selected GIProbe.*
+- **Bad:** *Compute global illumination for the selected GIProbe.*

engine/editor/index.rst

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+:allow_comments: False
+
+Contributing to the editor
+==========================
+
+.. toctree::
+   :maxdepth: 1
+   :name: toc-devel-editor
+
+   introduction_to_editor_development
+   creating_icons
+   editor_style_guide

engine/editor/introduction_to_editor_development.rst

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+.. _doc_introduction_to_editor_development:
+
+Introduction to editor development
+==================================
+
+On this page, you will learn:
+
+- The **design decisions** behind the Godot editor.
+- How to work efficiently on the Godot editor's C++ code.
+
+This guide is aimed at current or future engine contributors.
+To create editor plugins in GDScript, see :ref:`doc_making_plugins` instead.
+
+.. seealso::
+
+    If you are new to Godot, we recommended you to read
+    :ref:`doc_godot_design_philosophy` before continuing. Since the Godot editor
+    is a Godot project written in C++, much of the engine's philosophy applies
+    to the editor.
+
+Technical choices
+-----------------
+
+The Godot editor is drawn using Godot's renderer and
+:ref:`UI system <doc_user_interface>`. It does *not* rely on a toolkit
+such as GTK or Qt. This is similar in spirit to software like Blender.
+While using toolkits makes it easier to achieve a "native" appearance, they are
+also quite heavy and their licensing is not compatible with Godot's.
+
+The editor is fully written in C++. It can't contain any GDScript or C# code.
+
+Directory structure
+-------------------
+
+The editor's code is fully self-contained in the
+`editor/ <https://github.com/godotengine/godot/tree/master/editor>`__ folder
+of the Godot source repository.
+
+Some editor functionality is also implemented via
+:ref:`modules <doc_custom_modules_in_cpp>`. Some of these are only enabled in
+editor builds to decrease the binary size of export templates. See the
+`modules/ <https://github.com/godotengine/godot/tree/master/modules>`__ folder
+in the Godot source repository.
+
+Some important files in the editor are:
+
+- `editor/editor_node.cpp <https://github.com/godotengine/godot/blob/master/editor/editor_node.cpp>`__:
+  Main editor initialization file. Effectively the "main scene" of the editor.
+- `editor/project_manager.cpp <https://github.com/godotengine/godot/blob/master/editor/project_manager.cpp>`__:
+  Main Project Manager initialization file. Effectively the "main scene" of the Project Manager.
+- `editor/plugins/canvas_item_editor_plugin.cpp <https://github.com/godotengine/godot/blob/master/editor/plugins/canvas_item_editor_plugin.cpp>`__:
+  The 2D editor viewport and related functionality (toolbar at the top, editing modes, overlaid helpers/panels, …).
+- `editor/plugins/node_3d_editor_plugin.cpp <https://github.com/godotengine/godot/blob/master/editor/plugins/node_3d_editor_plugin.cpp>`__:
+  The 3D editor viewport and related functionality (toolbar at the top, editing modes, overlaid panels, …).
+- `editor/plugins/node_3d_editor_gizmos.cpp <https://github.com/godotengine/godot/blob/master/editor/plugins/node_3d_editor_gizmos.cpp>`__:
+  Where the 3D editor gizmos are defined and drawn.
+  This file doesn't have a 2D counterpart as 2D gizmos are drawn by the nodes themselves.
+
+Editor dependencies in ``scene/`` files
+---------------------------------------
+
+When working on an editor feature, you may have to modify files in
+Godot's GUI nodes, which you can find in the ``scene/`` folder.
+
+One rule to keep in mind is that you must **not** introduce new dependencies to
+``editor/`` includes in other folders such as ``scene/``. This applies even if
+you use ``#ifdef TOOLS_ENABLED``.
+
+To make the codebase easier to follow and more self-contained, the allowed
+dependency order is:
+
+- ``editor/`` -> ``scene/`` -> ``servers/`` -> ``core/``
+
+This means that files in ``editor/`` can depend on includes from ``scene/``,
+``servers/``, and ``core/``. But, for example, while ``scene/`` can depend on includes
+from ``servers/`` and ``core/``, it cannot depend on includes from ``editor/``.
+
+Currently, there are some dependencies to ``editor/`` includes in ``scene/``
+files, but
+`they are in the process of being removed <https://github.com/godotengine/godot/issues/53295>`__.
+
+Development tips
+----------------
+
+To iterate quickly on the editor, we recommend to set up a test project and
+:ref:`open it from the command line <doc_command_line_tutorial>` after compiling
+the editor. This way, you don't have to go through the Project Manager every
+time you start Godot.