Add tutorials for building VR applications

Cesium for Unreal has a handful of tutorials for building VR apps. For example, [here](https://cesium.com/learn/unreal/unreal-building-vr/) and [here](https://cesium.com/learn/unreal/vr-introduction/).

Cesium for Unity has VR _sample_ levels, but no formal tutorials for building VR apps. It'd be good to have at least an introductory tutorial for using Cesium in VR, if not multiple tutorials.

If people have any specific tutorials they'd like to see, feel free to respond to this issue with ideas!

Start by reading the linked Cesium for Unreal VR tutorials and inspecting the existing Cesium for Unity VR sample levels. Identify the introductory VR workflow the Unity documentation should cover and confirm how it differs from the samples. Done means at least one complete, tested introductory tutorial is added to the Unity documentation.

Written by the indexing model from the issue text.