update docs to be concise, and include purpose of docker, and setting up the frontend

- I wanted to emphasize that Docker is primarily to set up the backend + payments as a development environment. It will also set up the frontend to poke around, but Docker will not the recommended way to develop the frontend.
- Commonplace Fiddle is deprecated and will be removed
- I tried to make things more concise. Make it clear to contributors the paths they can go down, backend or frontend or both. Made the setup page more a gateway to the frontend/Docker docs.
- Removed some duplicate stuff from the setup docs vs the frontend docs
- Updated frontend docs to recommend WebIDE, and updated some commands.
- Added more repositories. Maybe it can be our canonicial list.

I'm touting this a lot, but I also plan on writing full frontend docs soon.

My RST sucks so check out if my links are busted.
We should add this to the -dist file https://github.com/mozilla/fireplace/blob/master/src/media/js/settings_local.js.dist