[BUG][SECURITY][8.19] Docs rule gap histogram (#7018)

* First draft

* Update rule-monitoring-dashboard.asciidoc

* Update docs/dashboards/rule-monitoring-dashboard.asciidoc

Author: nastasha-solomon

docs/dashboards/images/rule-monitoring-overview.png

@@ -32,6 +32,7 @@ The dashboard also includes data from all {kib} spaces. To display data only fro
 The following visualizations are included:
 
 * *Rule KPIs (key performance indicators)*: The total number of rules enabled, how many times they collectively ran, and their response statuses.
+* *Rule gap histogram*: The number of rules with unfilled gaps, filled gaps, and gaps that are being filled within the selected time range. Click the gap status labels—for example, **Rules with unfilled gaps**—to filter the histogram by the status you select.
 * *Executions by rule type*: Rule executions over time, broken down by rule type.
 * *Executions by status*: Rule executions over time, broken down by status.
 * *Total rule execution duration*: How long rules take to run, from start to finish.