Docs Updated for new Onboarding UI
This is a follow-up to https://gitlab.com/meltano/meltano/merge_requests/1359. It was already large so I broke up the docs updates into this issue:
- [x] contributing `.env` now required (already happening on hosted, need to do locally if not already)
- [x] getting started
- [ ] tutorials?
- [x] meltanodata.com instance setup
issue