This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
Lots of internal API changes and some docstrings were lost in the conversion.
I manually salvaged many of them but for all the rendering-related ones, an
additional pass is needed.
Added missing enum bindings in BaseMaterial3D and VisualServer.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
