Add documentation for Grafana Alerts

What does this MR do?

Our current documentation doesn't seem to discuss how to enable Grafana alerting (default: disabled). After the users configure Grafana, users can observe that the Alert tab is not readily available in their Grafana Dashboard. They need to set grafana['alerting_enabled'] to true and reconfigure their instance for this to be available.

Related issues

closes #6680 (closed)

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