[WIP] Revamp docs site

[rodjek]

The diff is unreadably large, so I recommend not trying to and instead pulling down the branch letting jekyll serve the site locally (cd docs; bundle install; bundle exec jekyll serve; open http://localhost:4000).

Important points:

• docs/ folder rather than gh-pages branch for easier updating
• New design
• Updated stale docs & added new docs for puppet-lint.rc and rake task.
• Check code comments rewritten as YARD
• Rake task rake docs:generate rebuilds the check documentation from the YARD comments
• Ignore the broken FAQ link for now, that's still a work in progress.

Over time, I'd like to move the rest of the comments over to using YARD and getting more of the docs being autogenerated, but this is a good start.