Do some cleanup of doc/index.md
I think a lot of people get to this landing page from `rustup doc`, and it's a bit tricky to find some useful information. I attempted to clean it up here a bit, from the commit message:
- Add quick link to API docs
- Add marker for external links, to help offline users
- Add information about using 'cargo doc' and the playground
- Clean up some of the wording
- Update body & header style to match rustdoc defaults
- Bump heading levels so main page header is 1, others are 2 (all were level 1 before)
Matthias Krüger
f840799385