Create documentation on metadata functionality

This should be a [cesium.com/learn](https://cesium.com/learn/) tutorial, and roughly the Unity equivalent of https://cesium.com/learn/unreal/unreal-visualize-metadata/.

Topics to cover:

- Accessing tileset metadata with C#
- Selecting a tileset feature during play and viewing its metadata properties

The tutorial should briefly cover a minimum C# setup for interaction (clicking the mouse to cast a ray from the center of the screen, for the purpose of building selection)