Define and document our backporting workflow with GitLab

Currently it is a bit unclear how that should happen, in gst-editing-services!5 (comment 75953) I just accepted to merge a MR targetting 1.14 and then forward ported it on master right away (which I guess is fine as soon as it is done with caution), but we should have a clear policy about that.

Once we have CI I would definitely want all commits go though it before merging, meaning also backports, we should define our worflow to make it as painless as possible.