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.
Yeah, I thought about how tightly coupled this library is with bedrock. If we wanted it decoupled, I don't think we'd want an `if` checking for `gaTrack`, as that would surely be unused bytes for external uses. Better idea would probably be to allow for custom callbacks, but then we'd have to write those in for each use in bedrock.

tl;dr - I'm not sure if we want to add complexity for the purpose of using this library externally.