Improved source code documentation

Godot 3.5, 4.2.2

Godot needs more detailed documentation on the source code of the engine, everything written here ([Engine core and modules](https://docs.godotengine.org/en/stable/contributing/development/core_and_modules/index.html)) is not enough, I have a decent amount of experience with C++, but I can't figure out which part of the engine is, some definitions are not clear to me, I have something to add to the engine, but the entry threshold is very complex, perhaps some kind of structure is needed that describes where the engine signal goes at a certain moment, during some action, in which file this code is located.

Perhaps we need a completely new section in the documentation describing the source code, this is necessary for programmers who want to contribute to the engine from around the world, as well as for those who write GDExtension extensions. Without knowing the source code I won't be able to write the extension, also this would be a great document for those who come to Godot from other companies like Google etc and need to understand in a short time how the code works.

It is clear that it is unlikely that anyone will describe every function inside the engine, but something needs to be done in this area.