add sla_miss_callback section to the documentation - #18305

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The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

close/reopen to rebuild

Awesome work, congrats on your first merged pull request!