Add a page describing what is GDNative

This closes #1024.

tutorials/plugins/gdnative/index.rst

@@ -5,5 +5,6 @@ GDNative
    :maxdepth: 1
    :name: toc-tutorials-gdnative
 
+   what_is_gdnative
    gdnative-c-example
    gdnative-cpp-example

tutorials/plugins/gdnative/what_is_gdnative.rst

@@ -0,0 +1,92 @@
+.. _doc_what_is_gdnative:
+
+What is GDNative?
+=================
+
+Introduction
+------------
+
+**GDNative** is a Godot-specific technology that lets the engine interact with
+native shared libraries at run-time. This means it can be used to run native
+code that was not originally supplied with the engine.
+
+GDNative is *not* a scripting language and has no relation to :ref:`GDScript <doc_gdscript>`.
+
+Differences between GDNative and C++ modules
+--------------------------------------------
+
+Both GDNative and :ref:`C++ modules <doc_custom_modules_in_c++>` can be used to
+run C/C++ code in a Godot project.
+Both GDNative and C++ modules also make it possible to integrate third-party
+libraries into Godot. Which one to choose depends on your needs:
+
+Advantages of GDNative
+^^^^^^^^^^^^^^^^^^^^^^
+
+GDNative is most suited to game logic (although GDScript/C# will still be more
+convenient for this use case). The easier setup for end users also makes
+GDNative more suitable for add-ons published to the :ref:`asset library
+<doc_what_is_assetlib>`.
+
+- GDNative is not limited to C/C++. Thanks to
+  :ref:`third-party bindings <doc_what_is_gdnative_third_party_bindings>`,
+  it can be used with many other languages.
+- The same compiled GDNative library can be used both in the editor and exported
+  project. With C++ modules, you have to recompile all the export templates you
+  plan to use if module functionality is required at run-time.
+- C++ modules are statically compiled into the engine. Every time you make a
+  change, you need to recompile the engine. Even with incremental builds, this
+  process often takes more than 10 seconds on most machines. In contrast, GDNative
+  only requires you to compile your library, not the whole engine.
+
+Advantages of C++ modules
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+:ref:`C++ modules <doc_custom_modules_in_c++>` are mainly recommended in cases
+where GDNative doesn't suffice for the reasons outlined below:
+
+- C++ modules provide deeper integration into the engine. GDNative is more
+  limited and can only access what the scripting API exposes (more or less).
+- C++ modules can be used to provide additional features in a project without
+  having to carry native library files around. This extends to exported projects
+  as well.
+- C++ modules are supported on all platforms. In contrast, GDNative isn't
+  supported on HTML5 and UWP yet.
+- C++ modules can be faster than GDNative, especially when lots of communication
+  through the scripting API is involved.
+
+Supported languages
+-------------------
+
+Official
+^^^^^^^^
+
+- C++ :ref:`(tutorial) <doc_gdnative_cpp_example>`
+- C :ref:`(tutorial) <doc_gdnative_c_example>`
+
+There are no plans to officially support additional languages with GDNative.
+That said, the community offers several bindings for other languages (see
+below).
+
+.. _doc_what_is_gdnative_third_party_bindings:
+
+Third-party
+^^^^^^^^^^^
+
+The bindings below are developed by the community and are sorted by alphabetical order.
+
+.. Binding developers: Feel free to open a pull request to add your binding
+.. if it's well-developed enough to be used in a project.
+
+- `D <https://github.com/godot-d/godot-d>`__
+- `Kotlin <https://github.com/utopia-rise/godot-kotlin>`__
+- `Nim <https://github.com/pragmagic/godot-nim>`__
+- `Python <https://github.com/touilleMan/godot-python>`__
+- `Rust <https://github.com/godot-rust/godot-rust>`__
+
+.. note::
+
+    Not all bindings mentioned here may be production-ready. Make sure to
+    research options thoroughly before starting a project with one of those
+    bindings. Also, double-check whether the binding is compatible with the
+    Godot version you're using.