docs: index: Add a quick link to Submitting patches

docs: index: Add a quick link to Submitting patches

We still get MRs in Gitlab from individual contributors on a regular
basis which in some ways just makes maintainer's or reviewer's life
just a bit more complicated. This ultimately means our guidelines are
probably not visible enough on the main page
(or some people wouldn't read them anyway). While this patch can't make
the problem go away, it can at least attempt to mitigate it by creating
a quick link to the 'hacking' page, skipping a lot of TL;DR contents
in contributing.rst which we link from the main page.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>