Consider moving docs site to VitePress

[cassidyjames]

VitePress is a nice Markdown-based static docs site generator, and we've used it now for the HMA docs. I originally chose mdBook because it was a single binary and very, very fast to get up and running with, but it might be nice to have a bit nicer-looking docs site which I think VitePress could give us.

We'd need to:

• rework the GitHub Actions workflow (which I already want to do) Simplify mdbook workflow, rebuilding on each deploy #451
• ensure existing mdBook docs URLs still work, i.e. w/versioned docs
• automate as much as possible, e.g. sidebar layout so we don't have to manually change things

Basically, I'd like the human workflow for docs to remain as close to what we have as possible, or better:

• docs are stored as Markdown files in the current structure
• new pages are simple to add either just by adding a file, or adding a file and updating a list somewhere
• reordering pages is as simple as reordering a list or renaming the files
• we don't have to manually duplicate content between multiple places to keep it in sync

I'll note that I'm not super concerned about the docs site itself being more of a flashy "marketing" landing page, as we have roost.tools/coop for that; but we can ensure it looks nice. :)

[ThisIsMissEm]

For versioned docs in vitepress, there's https://viteplus.github.io/versions/guide/

You can also define a set of redirect URLs iirc (but I think they may require a server or something, I recall having problems with them on github pages)

As for "flashiness", I'd say you probably want your docs to have some branding for Roost / Coop, but not go over the top.