Improve markdownlint job time by only scanning changed files
As mentioned in !6038 (merged), the markdownlint job typically takes 2:30. This can likely be cut down to less than half if we only run markdownlint on the files that have been changed.
We should be able to use something similar to what our product docs do: https://gitlab.com/gitlab-org/gitlab/-/blob/master/scripts/lint-doc.sh#L157