Style guides should be moved into a folder

[rdworth]

Instead of /js-style-guide/ and /html-style-guide/ we should have a top-level /style-guide/ with /js/ and /html/ inside it. In the future, perhaps we'd add a /css/. This also allows us to have a single style-guide landing page that introduces the concept of sharing all the guides across all foundation projects, and would link to all. It's also consistent with learn.jquery.com/style-guide/