Add RMM User Guide

[bdice]

Description

Draft only, not ready for review.

This is a placeholder. I plan to revise this heavily and cut up the new docs into multiple PRs.

Checklist

• [ ] I am familiar with the Contributing Guidelines.
• [ ] New or existing tests cover these changes.
• [ ] The documentation is up to date with these changes.

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[bdice]

I've broken this PR into a few parts that are ready to merge:

• #2137
• #2138
• #2139

After #2137 merges, I will start breaking up the new user guide documents into their own PRs.