Revert "change entrypoints to MDN"

This reverts commit dd03fd29f513224d04ae137ba8354fffdf46c60c.

The consensus in [the dev-mdn thread](https://groups.google.com/d/msg/mozilla.dev.mdn/-7KsfLqAPM8/J6lT7LH0-fgJ) was to host this at a separate domain. So, I filed [bug 1050458](https://bugzilla.mozilla.org/show_bug.cgi?id=1050458) to register and pick a new domain for this.