WIP: refactor navigation (significant change)

Our current structure has gotten a little chaotic, so here's a proposal for how it might look.

Most of the refactor has been done only in mkdocs.yml (ie, most of the actual docs are still in the same directory structure, but the navigation has been moved around). This will probably want to be gradually adjusted so that the source doc structure reflects the navigation.

At a couple of points (eg FAQs and Policies), pages naturally fit into two places, so in this initial version, I've repeated those docs in the navigation yaml. This has the annoying side-effect of jumping you to a different location when you select one of those pages, so I think we need a better solution before merging this.

I have a build of it up at https://portal.nersc.gov/project/mpccc/sleak/userdocs2/ to make it easy to explore and look for things to fix, etc.

This is a significant change, and I suspect a few things will be broken - the MR might need a few revisions to be ready. I've set a 5-approvals + brandon requirement to prevent it from going live before we're happy with it