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, so before the change, you could put a single tab directly inside the accordion wrapper. If you added a sibling tab, things would break (as the tab container controls the state). It worked if you followed the inherent/implied restriction, but seemed like a rare and brittle use case. Figured it would be better to just force each tab to have its own container.