[docs] API links are broken

[ncalexan]

I think the initial landing of the docs/ directory and Jekyl integration landed with version numbers, but the landing page at https://mozilla.github.io/mentat/ is full of broken links because the version numbers are missing.

I'd like to see:

• the links corrected
• some kind of "latest" and "v0.x" incorporated
• version numbers of the APIs to be at the top-level, like mentat/0.7/{swift,rust,...} rather than mentat/{swift,rust}/0.7
• version numbers to agree with the main Mentat version number (so 0.7 throughout, or whatever is appropriate)
• the stanza bundle exec jekyll serve --incremental somewhere in the documentation itself

@fluffyemily can you take some or all of those items? Thanks! (P.S. Your instructions worked well for me locally, and the site looks good!)

[ncalexan]

Also, I see

docs/.jekyll-metadata
docs/_site/

that need to be added to the top-level .gitignore.