Doc: update multiuser mode documentation

[lstein]

Summary

This PR updates the multiuser mode documentation to describe the user interfaces for adding, modifying and deleting users. It also removes some sections that were redundant with other parts of the documentation.

Related Issues / Discussions

QA Instructions

1. After pulling, run make docs and browse to Multi-User Mode (http://localhost:8080/InvokeAI/multiuser/user_guide/).
2. Read through.
3. Check for coherency, accuracy, broken links, etc.

Merge Plan

Simple merge

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[dunkeroni]

Something something Git permissions and LFS budget restrictions prevents me from pushing a commit with changes to your fork, so you get suggestions for each typo instead.

[lstein]

Something something Git permissions and LFS budget restrictions prevents me from pushing a commit with changes to your fork, so you get suggestions for each typo instead.

Whoa, sorry! I thought I had the "allow commit to repository" checkbox checked.

[dunkeroni]

Whoa, sorry! I thought I had the "allow commit to repository" checkbox checked.

No, it's something sillier than that. From what I can tell, it's because the repo uses LFS, so pushing makes a check for LFS changes even if the commit doesn't end up changing any. I as a maintainer could push to branches in PR, but I as a user cannot trigger LFS actions (which might pull from a budget) on your private repo without being a collaborator. The check for LFS counts, so I get hit by a cryptic permissions issue instead. Git has had some open issues about the situation for a few years now.