Renaming of gitlab-monitor to gitlab-exporter broke links

Problem to solve

After !16511 (merged), links broke because the redirect was not created in the old file. Additionally, it would be helpful to put a note in the docs explaining that GitLab Monitor was renamed to Exporter, so that searches for Monitor can still find the Exporter page.

I'm not sure if there were any disqus comments. I will see if I can find that out.