Resolve "Docs lint passes if a relative link starts with /doc"
What does this MR do?
Fix an edge case where the docs check linter would not catch a dead link.
I tested with the same link that was introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24339, and Nanoc correctly caught it https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/150139675.
It works!
What are the relevant issue numbers?
Closes #56763 (closed)