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.
Hmm, actually I see the lib is also using the global `trans` function too (as well as `gaTrack` elsewhere). It's up to you if you want to go this far in this PR. We could always file another bug if we think it's worth it.