[Docs] Update Jekyll + more

This started as a Jekyll update, then snowballed a bit. Most of the actual content changes here are in bourbon, which is noisy but ultimately not important. The rest of the changes don't result in much - a small change to how code blocks are formatted (just the wrapper really) and less boilerplate. SASS compilation is now handled in Jekyll so we don't have to remember to run a different command there to update.

Since this is a change though, people building docs (@vjeux, @spicyj, @chenglou) will need to make sure they update their installed gems (`bundle install`). We should only be running ruby stuff via `bundle exec ____` to make sure we get the version detection built in. I think I made the mistake recently of updating gems but not generating the site via bundle, which added some noise.