Backport What is GDNative? page to the `3.3` branch

This is needed to fix the build following the cherry-pick of
4709121d02d79b93f46d6cc726211d92b60fdad0 to the `3.3` branch.

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 +1,95 @@
+.. _doc_what_is_gdnative:
+
+What is GDNative?
+=================
+
+Introduction
+------------
+
+**GDNative** is a Godot-specific technology that lets the engine interact with
+native `shared libraries <https://en.wikipedia.org/wiki/Library_(computing)#Shared_libraries>`__
+at run-time. You can use it to run native code without compiling it with the engine.
+
+.. note:: GDNative is *not* a scripting language and has no relation to
+          :ref:`GDScript <doc_gdscript>`.
+
+Differences between GDNative and C++ modules
+--------------------------------------------
+
+You can use both GDNative and :ref:`C++ modules <doc_custom_modules_in_c++>` to
+run C or C++ code in a Godot project.
+
+They also both allow you to integrate third-party libraries into Godot. The one
+you should choose depends on your needs.
+
+Advantages of GDNative
+^^^^^^^^^^^^^^^^^^^^^^
+
+Unlike modules, GDNative doesn't require compiling the engine's source code,
+making it easier to distribute your work. It gives you access to most of the API
+available to GDScript C#, allowing you to code game logic with full control
+regarding performance. It's ideal if you need high-performance code you'd like
+to distribute as an add-on in the :ref:`asset library <doc_what_is_assetlib>`.
+
+Also:
+
+- GDNative is not limited to C and C++. Thanks to :ref:`third-party bindings
+  <doc_what_is_gdnative_third_party_bindings>`, you can use it with many other
+  languages.
+- You can use the same compiled GDNative library in the editor and exported
+  project. With C++ modules, you have to recompile all the export templates you
+  plan to use if you require its functionality at run-time.
+- GDNative only requires you to compile your library, not the whole engine.
+  That's unlike C++ modules, which are statically compiled into the engine.
+  Every time you change a module, you need to recompile the engine. Even with
+  incremental builds, this process is slower than using GDNative.
+
+Advantages of C++ modules
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+We recommend :ref:`C++ modules <doc_custom_modules_in_c++>` in cases where
+GDNative isn't enough:
+
+- C++ modules provide deeper integration into the engine. GDNative's access is
+  limited to what the scripting API exposes.
+- You can use C++ modules to provide additional features in a project without
+  carrying native library files around. This extends to exported projects.
+- C++ modules are supported on all platforms. In contrast, GDNative isn't
+  supported on HTML5 and the Universal Windows Platform (UWP) yet.
+- C++ modules can be faster than GDNative, especially when the code requires a
+  lot of communication through the scripting API.
+
+Supported languages
+-------------------
+
+The Godot developers officially support the following language bindings for
+GDNative:
+
+- C++ :ref:`(tutorial) <doc_gdnative_cpp_example>`
+- C :ref:`(tutorial) <doc_gdnative_c_example>`
+
+.. note::
+
+    There are no plans to support additional languages with GDNative officially.
+    That said, the community offers several bindings for other languages (see
+    below).
+
+.. _doc_what_is_gdnative_third_party_bindings:
+
+The bindings below are developed and maintained by the community:
+
+.. 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.
+.. Please keep languages sorted in alphabetical order.
+
+- `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.
+    Also, double-check whether the binding is compatible with the Godot version
+    you're using.