The goal here is to give newcomers an easier overview of the contributor
guide. Currently, the index page only points at a couple of sections in
the contributor index, which may be confusing. So:
1) Expand the contributor reference from the index page one more level.
2) Update headings in the contributor guide to match the toctrees and
their expected level.
3) Expand toctrees in the contributor index one more level.
4) Move references to the development enviroment to a higher level
toctree to make them visible in the index.
5) Apply consistent upper case heading.
Change-Id: Ifb9fdc96b368095437771217090120e83eaa0fa7
a36fd00951