Add docs on Gitflow merge request pipeline workflow

What does this MR do and why?

This adds documentation on Git Flow-style workflow rules.

The MR pipeline workflow is great for getting more context in MRs, but won't really work if you use more than just the default branch as a stable target - and many organizations and larger open source projects follow a more gitflow-like workflow with multiple branches (example: Ansible).

If you have master/main and devel branches, for example, you don't always have a merge request open from devel, and so no pipelines are triggered there.

So I thought Gitflow is quite well known and adding a simple template based on that will cover a lot more complex workflows without getting into complicated patterns for branching conventions - this just assumes that a long-lived branch is protected. Otherwise, you get into the discussion of devel/develop/development/main/master/trunk etc. I did still add tags explicitly, like in MergeRequest-Pipelines, in case people don't use protected tags.

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