docs: Create minimal template for docs MRs

Description

Proposes a minimal MR template for documentation MRs. I'm using the CLI project as a cozy place to figure out what we truly need, before propagating the changes out to the various repos managed by the groupeditor extensions group.

@eread, I'm adding you as a reviewer because you 1) do a lot of work for Gitaly and 2) are good at spotting when I've run ahead of myself and need to fill in details. This MR is an end-of-day thought and I guarantee it's missing details. The template deserves the usual balance of thoroughness and brevity.

And yes, if we can settle on something, I'll add it to our TW team docs somewhere. This need will come up again; it's just more glaring now that I'm seeing all the repos for groupeditor extensions.

Other documentation MR templates for awareness / consideration:

• gitlab repo: https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/merge_request_templates/Documentation.md
• gitlab-docs repo (cognate, not match): https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.gitlab/merge_request_templates/Default.md?ref_type=heads
• Probably some others I haven't found

Related Issues

How has this been tested?

It hasn't!

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap