docs: Link main "contributing" from programming-specific guide

docs: Link main "contributing" from programming-specific guide

There are two guides to contributing: `hacking.rst` is focused on code
contributions, and `contributing.rst` is more general. Clarify scope of
`hacking.rst` and link to the general guide in its references.

Signed-off-by: Tim Small <tim@seoss.co.uk>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>