Add link path test to markdownlint

What does this MR do?

Add a markdownlint rule to catch links that ../ out of the current directory then back in

• First run: https://gitlab.com/gitlab-org/gitlab/-/jobs/9458841131
  • Summary: 219 error(s)
• Second run: https://gitlab.com/gitlab-org/gitlab/-/jobs/9478456284 (after some cleanup in !185054 (merged))
  • Summary: 196 error(s)
• Third run: https://gitlab.com/gitlab-org/gitlab/-/jobs/9497748847 (after almost all links already fixed).
  • Summary: 2 error(s)
• Clean run: https://gitlab.com/gitlab-org/gitlab/-/jobs/9498233164 (after last two links fixed)
  • Summary: 0 error(s)

This also works with --fix to auto-fix issues, though it shouldn't really be needed now that everything is cleaned up and it'll be enforced in the pipeline, so any new issues should be directly related to the work you are doing. If needed though, you can use --fix or Quick fix in the on hover in VS Code.

With this new rule, we no longer need doc/.vale/gitlab_docs/RelativeLinksDoubleSlashes.yml, which was checking for both // and ../doc (which is now caught by the new rule). I've added the // check to doc/.vale/gitlab_docs/InternalLinkFormat.yml, which actually works better than before because it looks for // anywhere in the path, not just at the start.

Screenshot

In VS Code:

Related issues

Related to Docs: Clean up directory paths in docs (#517260)

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