Improve the Data paths page

- Add examples for the editor data paths and mention XDG compliance
- Add information about the self-contained mode

tutorials/io/data_paths.rst

@@ -15,16 +15,16 @@ A path like ``C:\Projects`` will become ``C:/Projects``.
 Resource path
 -------------
 
-As mentioned before, Godot considers that a project exists in any
-given folder that contains a ``project.godot`` text file, even if such
-file is empty.
+As mentioned in the :ref:`doc_command_line_tutorial`, Godot considers that
+a project exists in any given folder that contains a ``project.godot``
+text file, even if such file is empty.
 
 Accessing project files can be done by opening any path with ``res://``
 as a base. For example, a texture located in the root of the project
 folder may be opened from the following path: ``res://some_texture.png``.
 
 User path (persistent data)
--------------------------------
+---------------------------
 
 While the project is running, it is a common scenario that the
 resource path will be read-only, due to it being inside a package,
@@ -40,3 +40,48 @@ and ``%APPDATA%/Name`` on Windows. ``Name`` is taken from the
 application name defined in the Project Settings, but it can be
 overridden on a per-platform basis using
 :ref:`feature tags <doc_feature_tags>`.
+
+Editor data paths
+-----------------
+
+The editor uses different paths for user data, user settings and cache depending
+on the platform. By default, these paths are:
+
++---------------+---------------------------------------------------+
+| Type          | Location                                          |
++===============+===================================================+
+| User data     | - Windows: ``%APPDATA%\Godot\``                   |
+|               | - macOS: ``~/Library/Application Support/Godot/`` |
+|               | - Linux: ``~/.local/share/godot/``                |
++---------------+---------------------------------------------------+
+| User settings | - Windows: ``%APPDATA%\Godot\``                   |
+|               | - macOS: ``~/Library/Application Support/Godot/`` |
+|               | - Linux: ``~/.config/godot/``                     |
++---------------+---------------------------------------------------+
+| Cache         | - Windows: ``%TEMP%\Godot\``                      |
+|               | - macOS: ``~/Library/Caches/Godot/``              |
+|               | - Linux: ``~/.cache/godot/``                      |
++---------------+---------------------------------------------------+
+
+- **User data** contains export templates and project-specific data.
+- **User settings** contains editor settings, text editor themes,
+  script templates, etc.
+- **Cache** contains temporary data. It can safely be removed
+  when Godot is closed.
+
+Godot complies with the `XDG Base Directory Specification
+<https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html>`__
+on all platforms. Environment variables can be overridden as per
+the specification to change the editor (and project) data paths.
+
+Self-contained mode
+~~~~~~~~~~~~~~~~~~~
+
+If you create a file called ``._sc_`` or ``_sc_`` in the same directory as the
+editor binary, Godot will enable *self-contained mode*. This will make Godot
+write all user data to a directory named ``editor_data/`` in the same directory
+as the editor binary. This is useful to create a "portable" installation,
+which can then be placed on an USB drive.
+
+The `Steam release of Godot <https://store.steampowered.com/app/404790/>`__
+uses self-contained mode by default.