.github: add a "Contribution Guidelines" to the pull request template

.github: add a "Contribution Guidelines" to the pull request template

These guidelines refer contributors to the "Submitting Patches to Ceph" doc
and the "Submitting Patches to Ceph - Backports" doc. Even though there are
already tips for titling/signing commits in the PR template, these tips
are commented out and easy to gloss over once the contributor creates the
PR. These existing tips do not include any pointers about staging backports.

Fixes: https://tracker.ceph.com/issues/55418
Signed-off-by: Laura Flores <lflores@redhat.com>