Document texture combiner tool

[Cameron-Micka]

Describe the issue

We are missing conceptual documentation on the texture combiner tool ported from MRTKv2: #28

We should also address these concerns in the docs: microsoft/MixedRealityToolkit-Unity#8079

Feature area

All docs are missing.

Existing doc link

n/a

Additional context

Related: #13

[Cameron-Micka]

We should call out this Blender tool in the docs: https://twitter.com/HoloMotoRanger/status/1580210869728083968?s=20&t=BQgxrp6_Je49-J2IPTwRNw