Polish new MRTK3 Learning Paths Documenation

[AMollis]

This issue has been migrated a new MRTK repository, and the status of this issue will now be tracked at the following location:

• Polish new MRTK3 Learning Paths Documenation MixedRealityToolkit/MixedRealityToolkit-Unity#70

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Overview

The MRTK3 Learning Paths documents need to be polished.

Tasks

• Copy documentation changes from https://github.com/microsoft/MixedRealityToolkit-Learning-Paths-Partners to https://github.com/MicrosoftDocs/learn-pr
• Update https://github.com/microsoft/MixedRealityLearning/ to point to new MRTK3 pre-release asset packages (*.unitypackage)
• Fix any other bugs in documentation