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 is not the recommended way to develop the frontend.
- 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.