Heartbeat timeout docs

[karenbraganz]

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In Airflow 2.10, task logs on the Airflow UI do not use the terminology "zombie task" to describe when the heartbeat of a local task job times out. Instead, such events are recorded on the Event Log page as a "heartbeat timeout". In order to make troubleshooting such issues more intuitive to Airflow users, the terminology in the documentation should match that in the logs.

I have added more emphasis to the heartbeat timeout terminology in the documentation so that Airflow users are able to easily find this documentation when they see "heartbeat timeout" events and relate the information in the documentation to what they see in the logs.

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[RNHTTR]

I added a few suggestions, but this is a great start!