Basic docs covering all code around proof of concept VR branch

[Truemedia]

This issue is to piece together documentation that forms a base understanding of the code that works for displaying a basic VR immersive web experience that works on WebXR supported browsers (I will myself be testing directly against a Meta Quest 3 headset).

[Truemedia]

Just following up from our call, @alvarosabu to provide a section of the docs about the useXR function in a format similar to what was done for the post processing module.