Improve discoverability of Engine core and modules pages

[allenwp]

Your Godot version:
Godot 4

Issue description:
The Engine core and modules section of the docs provides information to experienced programmers who are new to Godot.

Personally, I have had a hard time finding and referring back to these pages because they are nested so deep that the pages do not appear on the sidebar. My instinct when looking for these pages is to look for items on the sidebar titled “Core types”, “Variant class”, or “Object class” under the engine development section. These pages do exist, but are nested deep enough that they do not appear on the sidebar.

My proposal is to remove the current intermediate Engine core and modules landing page and replace it with its two sub-pages. The new structure would look something like this:

• Engine development
  • Engine architecture
    • Engine core (and modules?)
      • Godot's architecture diagram
      • Common engine methods and
      • Core types
      • Variant class
      • Object class
      • Inheritance class tree
      • Internal rendering architecture
      • 2D coordinate systems and 2D
    • Extending Godot by modifying its source code
      • Custom modules in C++
      • Binding to external libraries
      • Custom Godot servers
      • Custom resource format loaders
      • Custom AudioStreams
      • Custom platform ports
      • Unit testing

I expect that no content would be lost with this new structure and promoting these leaf-level pages to the sidebar would make them more discoverable and faster to refer back to.

You can see the history of these menu items being buried across docs version 3.6, 4.0, and 4.1:

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/latest/contributing/development/core_and_modules/index.html

[Edit: Originally I referenced #5877 as an example of users having a hard time finding this documentation, but this issue is from before these menu items became hidden.]