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.
Good call! Then we could remove this section and add it as a comment to the settings file