Class docs

[RedMser]

Outstanding issues:

• Documentation of custom methods does not show up (e.g. TranslationFluent.add_bundle_from_text). -> Docs comments do not show up if only some of the class members/methods are documented godot-rust/gdext#810
• A single newline in the docs comment is turned into a paragraph in the XML docs (I assume [br]?). If we were to follow how Godot XML docs are written, how ## GDScript docs work, and how VSCode renders them, newlines should be ignored and only double-newlines should be treated as new paragraphs. -> Docs comments do not follow usual markdown newline rules godot-rust/gdext#811 (this is not a blocker)
• Unable to document Project Settings (is this even technically possible with what Godot's XML API provides?) -> Ignoring this for now, docs for settings stays in readme -> new issue Add class documentation to ProjectSettings #44
• Update according to API changes in Remove bundle list #41 and Add custom functions support #42

Fixes #2