Document gltf Uber extension strings

[twojtasz]

Strings here modules/gltf/src/gltf/gltf-builder.js must be known and documented to access this data.

[ibgreen]

Since these are common to both builder and parser we should probably have a separate doc uber-gltf-extensions.md that describes the extensions we implement and in cases where they are unique to us, their semantics.

We can reference this doc in the two other class-specific docs.

However, the idea was to implement addCompressedPointCloud and getCompressedMesh to hide the need to use the extension names from the app (but I understand that from a file format documentation point of view, it is desirable to document them, though I might consider them somewhat experimental at this stage).

[ibgreen]

Done