Add a bot comment to docs MRs that get a long pipeline

Problem

We've had several instances now where team members create docs-only merge requests that change one of the two pages mentioned in https://docs.gitlab.com/development/documentation/workflow/#pipelines-and-branch-naming -

• doc/_index.md
• doc/api/settings.md

Those pages are used in backend tests. TWs are caught off guard by the hard requirement for a pipelinetier-3 pipeline, and don't know why the long pipeline is required until someone points them to @marcel.amirault's documentation I linked in the first paragraph.

Proposal

Create an integration that posts a bot comment to a merge request that changes one of these files.