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.
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.
- Add a link to the audio generator demo in AudioStreamGenerator
and AudioStreamGeneratorPlayback.
- Mention that signed PCM8 data is expected in AudioStreamSample
(and how to convert unsigned PCM8 to signed PCM8).
