docs: Update hacking.rst

This organizes the existing contents into sections, tweaks some parts
a bit and adds links to the pages where the contents that were ripped
out of hacking.rst now live, either inline or in the catch-all "further
reading" section depending on what makes more sense.

The result is that it's now possible to consume this page, which is
the entry point for new contributors, in just a few minutes, and then
drill down further based on factors such as the familiarity with the
open source development model or mail-based workflows.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>