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.
I'd rather settle on adding a few extra bytes doing a type check than implementing callbacks for this (although callbacks could be much more useful for other things!). But agree this doesn't need to be part of this change.