Add support for built-in Godot documentation

[limbonaut]

Godot has a built-in documentation system that does not require online access. Godot game developers usually expect to find some useful information about the public API in there.

Documentation tool is part of Godot, e.g. godot --doctool. It generates XML files for the public API exposed to ClassDB, and also updates those XML files on subsequent runs, but it only deals with the document structure. The descriptions need to be added manually to those XML files. It preserves the added content on subsequent runs. There is also a tool to export sphinx-based RST-formatted class documentation.