docs: Add multiuser documentation and reposition Workflows

[lstein]

Summary

This PR migrates the multiuser documentation from docs-old into the new astro docs directory. It also moves the Workflows section into Features, which seems to be a more appropriate place for it than at the top level.

For convenience, this PR also updates the make docs Makefile target.

Related Issues / Discussions

QA Instructions

1. make docs will fire up astro
2. Browse the reorganized Features section.
3. Check for broken images and links.

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)

[joshistoast]

Good overall add, but needs some updates.

Notes:

• I added some quick suggestions for reordering to keep the non subgrouped docs links at the top of the respective parent group.

• Lots of areas where the <Steps> component can be used for those instructional step-by-step ordered lists.

• I think we should move the spec to the corresponding invoke dir as a feature readme, as opposed to being a docs page.

• Found a couple broken links and left the fix suggestions.

• The multi user docs leaves a lot of "not right now, but in the future for sure" promises, maybe it'd be better to just say "Not at this time, create an issue if this is important to you" verbiage?

• There seemed to be an instance or two of inconsistency between existence of a feature. For example the user vs administrative docs seem to have different opinions on the existence of a user management webui. And there is also documentation now of a board having a desciption- which unless I'm mistaken doesn't exist.

• I can have my LLM do another pass over this to see if I missed anything, but this was everything I found initially.

[lstein]

Thanks for the thorough review. There was a lot more cruft in the docs than I realized. I've implemented all the suggested changes and removed redundant and out-of-date content. Let me know if I missed anything.