New main navigation

- Changes the navigation for small screens to a footer anchor pattern (bottom of the page in vertical blocks on small screen, top of the page horizontal and inline on large screen)
- Menu link on small screens that anchors to bottom of page
- Future-proofing the nav (add pretty much as many nav items as you'd want to)
- Separating the nav from the footer and header structurally
- New styles for the nav items
- Updates to readme
- Fix for body "background" styling