Add class reference support with godot-cpp#1374.

[akien-mga]

Class reference copied from the 3.6 docs.

Depends on:

• Generate docs from GDExtensions using --gdextension-docs with --doctool godotengine/godot#91518
• Allow submitting documentation to the Godot editor godotengine/godot-cpp#1374

Which will be merged for 4.3, so merging this will make future builds of the extension 4.3+.