[DRAFT] Allow submitting documentation to the Godot editor

[dsnopek]

This depends on Godot PR https://github.com/godotengine/godot/pull/83747 to add support for submitting documentation to the Godot editor from a GDExtension.

I'm not entirely sure how we should expose this to developers. What I have here presently, is mostly about being able to test that the functionality from the Godot PR works. This could certainly use some design help for how to expose the build-system part (both for SCons and cmake) as well as how the developer should trigger the registration.

We'll also probably need some tooling somewhere, to assist in generating the XML files for extension classes. It looks like godot --doctool doesn't include extension classes, so we'll either need to modify it so that it does, or come up with some other process.