Improve docs site landing pages/versioning experience

[cassidyjames]

Right now we use effectively the same mdbook workflow to build versioned docs sites for both Coop and Osprey:

• Workflow: Coop, Osprey
• Docs source: Coop, Osprey
• Docs index: Coop, Osprey
• Latest docs: Coop, Osprey

The versioning is a little awkward UX-wise:

• The index page doesn't look great; it's barebones and looks like it's not meant for humans 😅
• When in the docs, it's not obvious which version you're browsing without looking at the URL
• When browsing old docs, it's not obvious you're not browsing the latest (which might be on purpose, but you should be doing it consciously!)
• /latest is the latest unreleased docs; there's no /stable for the latest release

I don't mind the mdbook-generated sites themselves, but I think we could improve this with a few changes:

• Add a /stable symlink to the latest released version of the docs
• Add the list of versions to the built mdbook site itself (i.e. a list on the index and/or a drop-down in the nav bar) instead of a standalone index page
• Redirect / to either /stable or /latest (I could be convinced either way)
• Consider adding an alert bar to old docs to flag that they're not the latest version, and to /latest docs to flag that they're for the unreleased version