remove current HTML-based symbol, add any missing aria-* attributes

[skip sauce]

Use the structure as documented in
http://v4-alpha.getbootstrap.com/components/navbar/#collapsible-content
- wrapping both the toggle and the navigation in a `<nav>`, using a
`<ul>` for the navigation items

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color

- drop the index pages save for examples
- comment out the extend section for now
- fix up spacing fix on team list