[SECURITY][ResponseOps][Alerting][8.19] Alert Deletion Docs (#6952)

* First draft

* Update docs/detections/alerts-ui-manage.asciidoc

Author: nastasha-solomon

docs/detections/alerts-ui-manage.asciidoc

@@ -308,3 +308,9 @@ is `host.name: "Windows-ArsenalFC"`.
 
 NOTE: Refer to <<timelines-ui>> for information on creating Timelines and Timeline
 templates. For information on how to add Timeline templates to rules, refer to <<rules-ui-create>>.
+
+[discrete]
+[[clean-up-alerts-sec]]
+=== Clean up alerts
+
+Manage the size of alert indices in your space by clearing out alerts that are older or infrequently accessed. You can do this by {kibana-ref}/view-alerts.html#clean-up-alerts[running an alert cleanup task], which deletes alerts according to the criteria that you define.