A reworked process/index.rst

The process book is arguably the most important documentation we have; the
top three trafficked pages on docs.kernel.org are found here.  Make a
beginning effort to impose a more useful organization on this page to ease
developers into the community.

Acked-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>