Added documentation explaining difference deferred vs async operators

[dabla]

Added documentation explaining difference deferred vs async operators, related to the PR #60268 which added native async support for PythonOperators.

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[jscheffl]

Looks okay for me, some proposals as feedback.

[dabla]

Thanks for the thorough review @jscheffl will fix the remarks.

[kaxil]

Doc review of the new "Deferred vs Async Operators" page. The content fills a real gap — users need this guidance. Most comments are about broken cross-references (build failures) and making the examples more complete for users.

[uranusjr]

I feel we should also discuss when you should not use one of the other. For example

• You probaly shouldn’t prefer async over defer if the wait can be long (since an async operator is kept running in a process, which can waste resource).
• You probably shouldn’t use defer if deferring happens many times and are generally short (even if you write a custom deferrable operator) since it stops the process every time it defers and restarts everything it resumes, which can introduce a lot of overhead.

Also, the mention about existing deferrable operator can be a bit misleading IMO. If your use case is better for defer, and there’s no existing deferrable operator available, your best course would be to write one yourself, not to use async. It should depend more about the use case, not what Airflow has available.

[jscheffl]

Looks good for me now!

Feedback by others welcome though.