This allows not accounting for certain visible meshes during baking
(such as foliage and thin fixtures).
This also adds a clarification about transparent materials always
being excluded in the OccluderInstance3D documentation.
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.
GPUParticles attractors and collision are currently only available in 3D.
Their 2D counterparts haven't been implemented yet, but they will use
separate nodes.
