Move start_guide.md to docs/ and delete using_tox.md

`start_guide.md` is virtually an exact duplicate of `docs/using_tox.md`, so one of them should be removed (I arbitrarily chose `using_tox.md` in favor of the small improvements that `start_guide.md` has). I also moved the start guide to `docs/`.

Understandably, the guide does need more work, but I wanted to focus on removing this redundancy first.

/cc @pflo (created `start_guide.md`)