Rewrite docs with Doctocat

[colebemis]

This pull request refactors the Primer CSS documentation site to use Doctocat. The new site has feature parity with the old site (primer.style/css).

TODO

• Remove old docs
• Update contributing docs
• Make sure documentation is up to date
• Test in Chrome
• Test in Firefox
• Test in Safari
• Test in Edge

[vercel]

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[colebemis]

Safari: Nav links in the nav drawer aren't clickable if they are below the fold.

I spent most of yesterday trying to fix this bug. Turns out there is no straightforward fix. After discussing with @emplums, I think it's okay if we move forward with this pull without fixing this since it only affects macOS Safari users with a viewport of <1000px.

I filed an issue for this bug on the primer/dococat repo (primer/doctocat#56) and react-focus-lock repo (theKashey/react-focus-lock#79).

[shawnbot]

This is a huge amount of great work and after clicking around on the site it seems to do everything at least as well (if not better) than our existing site. Search works great. Prefetching seems to be working nicely and page transitions are practically non-existent. And it all looks really good, down to the improved syntax highlighting. I'm sure we'll find issues, but with Doctocat in place I think they'll be a lot easier to fix than they were before. Kudos, @colebemis! 👏