Add support for split class reference

[YuriSizov]

Related to https://github.com/godotengine/godot-proposals/issues/4543.

This adds support for several groups of classes in the class reference. The classes are grouped based on their filename prefix, which is implemented by an accompanying PR in the main godot repo (https://github.com/godotengine/godot/pull/63497).

This PR does not include the changed class files themselves, so the 3 categories there would be empty upon merging, until we sync the documentation with current master.

The end result is this:

https://user-images.githubusercontent.com/11782833/181056676-a9896f25-16ce-4b10-99d5-63a2cb7a5634.mp4