Refine and consolidate merge request templates for project

From recent experience of reviewing code MRs from @sarahgerman and also participating in in %15.1 and %15.2 (to a lesser extent) docs releases, I thought we could give our MR reviewers and authors more guidance. I've noticed @oregand sort-of creates his own checklist when he reviews code, so we should formalize something like that.

While here, I thought we might cut down on the number of MR templates we use, because they should be able to be relatively generic.

CC @gl-docsteam FYI, because we all have a stake in gitlab-docs merge requests being authored and reviewed well.