Fix the links to the Celery documentation.
Review Request #12382 — Created June 17, 2022 and submitted — Latest diff uploaded
The URL for the Celery site has changed. For a while, the old domain was
unavailable, though they've since reclaimed it and redirected it to the
To be on the safe side, we now use the new domain in all of our links.
Checked all the links to ensure they worked.