Add a new section to the Troubleshooting monitoring section to tell users what to do when the KubePersistentVolumeFillingUp alert fires.

This is a topic that often comes up for customers, so adding this info to the docs should help reduce the load on Support.

More information

• in this KCS article: https://access.redhat.com/solutions/7033177
• in this runbook: https://github.com/openshift/runbooks/blob/master/alerts/cluster-monitoring-operator/KubePersistentVolumeFillingUp.md