Add comment about the structure of layer.json in CesiumTerrainProvider

`CesiumTerrainProvider` is where CesiumJS parses `layer.json` and `quantized mesh` to render terrain. However, currently, we only have the documentation for the quantized mesh [here](https://github.com/CesiumGS/quantized-mesh). The structure of `layer.json` seems to be undocumented and there is an implied deprecated feature in there too. For example, the `available` field should be removed from the `layer.json` and embedded in the metadata extension of the quantized mesh. Adding some comments about its structure can help developers debug what's wrong with their dataset or code without having to look deep into the source code

Start at CesiumTerrainProvider, where CesiumJS parses layer.json and quantized mesh, and compare the implementation with the existing quantized mesh documentation. Document the layer.json structure in comments, including the implied deprecation of available in favor of the quantized mesh metadata extension. Done means developers can understand and debug the dataset structure without tracing the source.

Written by the indexing model from the issue text.