docs: clarify Redis maintenance for CeleryExecutor

[Will-thom]

Closes #55215.\n\nAdds a Redis maintenance section to the CeleryExecutor docs explaining what Airflow stores in Redis when it is used as the Celery broker, how that differs from the metadata database, when cleanup is appropriate, and why Redis should not be flushed while schedulers or workers are running.\n\nValidation:\n- git diff --check -- providers/celery/docs/celery_executor.rst\n- rg -n Redis maintenance providers/celery/docs/celery_executor.rst

[kaxil]

What's the motivation behind adding this?

[Will-thom]

The motivation is to make the Redis maintenance boundary explicit for CeleryExecutor users.

The linked issue asks what Airflow stores in Redis and whether it is safe to clean it. Today the docs explain Redis as the broker, but they do not clearly distinguish broker-side transient Celery state from the Airflow metadata database, nor warn that flushing Redis while schedulers/workers are active can drop queued work or make task state inconsistent.

This section is meant to prevent operators from treating Redis cleanup like airflow db clean: metadata cleanup and broker cleanup affect different stores and need different safety conditions.

[github-actions]

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[potiuk]

Docs-only — accurate Redis-broker maintenance guidance for the Celery executor. Verified the diff. LGTM.

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.