Define and document process for bypassing merge train for urgent site updates

NOTE: This is a placeholder issue for the topic discussed in #6213 (closed), it needs to be fleshed out.

See also this slack thread: https://gitlab.slack.com/archives/CHLKE258E/p1588016641067300

Current Status

• This issue is preventing proper usage of the "Merge Immediately" dropdown. Currently, you have to wait for at least one pipeline to complete successfully before you can pick "Merge Immediately". Here's a summary of the workaround to use the "Merge Immediately" dropdown:
  1. NOTE: Adding [SKIP REVIEW APP] to the MR title before the pipeline starts can save some time.
  2. Start at least one pipeline via a commit. Do not add to the merge train.
  3. Let the pipeline finish (do not cancel it, you have to wait for it to finish successfully)
  4. When the pipeline finishes, you will get the "Merge Immediately" dropdown once you resolve the thread.
• Alternately, we can merge to master immediately, using this process:
  1. Be logged on as a maintainer
  2. In repository settings, add yourself to the list of users allowed to merge to master
  3. Locally, pull the latest master and branch to merge.
  4. git merge --no-ff <branch-name>
  5. git push
  6. Remove yourself from the list of users allowed to merge to master
  7. Watch the master pipeline and ensure the master build for the commit passes and there are no problems.