The merge request workflow can be confusing to people unfamiliar with
it, so add screenshots.
Also add a new section on how to revise merge requests, since a lot of
people tend to open new merge requests to make any changes.
Eliminate the separate "How to Prepare a Merge Request for Submission"
section -- merge it with the main section.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1367>
