WIP: Improve user documentation for aks-desktop features

AKS Desktop features should be documented for users.

This is to track which features need documentation added, and which documentation has been user tested with real people.

Documentation should be here: https://learn.microsoft.com/en-us/azure/aks/aks-desktop-overview

- [ ] need to get a complete list of features that need documentation
- [ ] make list of gaps in documentation
- [ ] plan user testing sessions with internal folks

List of features?
- [ ] projects
- [ ] Logging into existing cluster
- [ ] ...

Note:
- [ ] Ask PMs about this

Start by reviewing the AKS Desktop feature list and the existing user documentation at https://learn.microsoft.com/en-us/azure/aks/aks-desktop-overview. Identify documentation gaps, ask PMs for the complete feature list, and plan user testing with internal people; done means the feature inventory, gap list, and testing plan are complete.

Written by the indexing model from the issue text.