Add docs for mozilla-accordion.js. Bug 1007717.

- Add GA tracking for accordion collapse event.
- Move ARIA from HTML to JS.
- Refactor mozilla-accordion.less to rely on new
  (non-ARIA) HTML hooks.
- Disallow direct child tabs - all accordion tabs/panels
  must be in a separate wrapper element.
Poor heading level 6 feeling left out? :trollface: