Docs: track improvements to monorepo pages

Problem to solve

I recently discovered the Gitaly team had added a set of pages to https://docs.gitlab.com/user/project/repository/monorepos/. The concepts in these pages blur the lines between groupgitaly and devopscreate, and they need a heavy rewrite to match our CTRT format.

That sounds like tw-leadcreate work, and it's work I know how to do! Let's track the work.