Add a FAQ item about the UI toolkit used by Godot

(cherry picked from commit 23ecee304b666dd356362f57d247a72915b8f4e2)

about/faq.rst

@@ -355,6 +355,8 @@ developer experiences as a whole.
 Bonus points for bringing screenshots, concrete numbers, test cases, or example
 projects (if applicable).
 
+.. _doc_faq_non_game_applications:
+
 Is it possible to use Godot to create non-game applications?
 ------------------------------------------------------------
 
@@ -372,6 +374,8 @@ Check out `Material Maker <https://github.com/RodZill4/material-maker>`__ and
 `Pixelorama <https://github.com/Orama-Interactive/Pixelorama>`__ for examples of
 open source applications made with Godot.
 
+.. _doc_faq_use_godot_as_library:
+
 Is it possible to use Godot as a library?
 -----------------------------------------
 
@@ -385,6 +389,26 @@ engine instead. Keep in mind rendering engines usually have smaller communities
 compared to Godot. This will make it more difficult to find answers to your
 questions.
 
+What user interface toolkit does Godot use?
+-------------------------------------------
+
+Godot does not use a standard :abbr:`GUI (Graphical User Interface)` toolkit
+like GTK, Qt or wxWidgets. Instead, Godot uses its own user interface toolkit,
+rendered using OpenGL ES or Vulkan. This toolkit is exposed in the form of
+Control nodes, which are used to render the editor (which is written in C++).
+These Control nodes can also be used in projects from any scripting language
+supported by Godot.
+
+This custom toolkit makes it possible to benefit from hardware acceleration and
+have a consistent appearance across all platforms. On top of that, it doesn't
+have to deal with the LGPL licensing caveats that come with GTK or Qt. Lastly,
+this means Godot is "eating its own dog food" since the editor itself is one of
+the most complex users of Godot's UI system.
+
+This custom UI toolkit :ref:`can't be used as a library <doc_faq_use_godot_as_library>`,
+but you can still
+:ref:`use Godot to create non-game applications by using the editor <doc_faq_non_game_applications>`.
+
 Why does Godot not use STL (Standard Template Library)
 ------------------------------------------------------