RFC: Changes to SubmittingPatches.rst

For a long time now I've been planning to refactor SubmittingPatches.rst, and this week I finally got to doing it. The PR - https://github.com/ceph/ceph/pull/30705 - is open and I'm hereby soliciting reviews and comments on it. I'd especially like to call your attention to the new SubmittingPatches-backports.rst section, where I have attempted to "codify" (and explain the reasoning behind) the current state of backporting best practices that the Ceph project has developed over the years. If you are currently, or plan to be, involved in backporting patches to the stable branches in the Ceph git repo, your input is most welcome. Just add comments/reviews to the PR. Thanks, Nathan -- Nathan Cutler Software Engineer Distributed Storage SUSE LINUX, s.r.o. Tel.: +420 284 084 037