Fix broken links to celery documentation

[davidhagens]

The celery documentation has been moved from https://docs.celeryproject.org/ to https://docs.celeryq.dev/. The old links now refer to a 404 error page, the new links to the actual documentation.

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[boring-cyborg]

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[eladkal]

(Not sure if should be done in same PR but we need also to revert #22254 )

[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest main or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!