docs.edx.org - Visual Styling/FED Revamp

This work is a quick visual revamp to the docs.edx.org work previous done by @markchang and @jkarni. It includes:

* a revamp of the visual elements of the page
* a more organized set of page assets (everything is now in a edx-docs subdirectory to not cause confusion with the rest of the repo)
* a revamp to the content presented in the page (after talking with @jkarni): multiple links to specific doc pages are now suggested under each audience; a place to introduce what these docs cover/don't cover; sidebar/support content
* a revamp of the FED assets and buildout (using Bourbon/Neat/Font Awesome with a BEM-philosophy to the HTML and semantics)
* the layout is now responsive and breaks for small desktop, tablets, and mobile views

This page still needs copy and a quick browser proofing review.