docs: Add requirements for prometheus-operator
Fixes:
# Prometheus Operator
+## Requirements
+
+- A Kubernetes cluster with a
+[PersistentVolume](https://kubernetes.io/docs/concepts/storage/persistent-volumes/)
+plugin, e.g. [OpenEBS](/docs/components/openebs/openebs.md) or one of the
+[built-in](https://kubernetes.io/docs/concepts/storage/persistent-volumes/#types-of-persistent-volumes)
+plugins.
+
+>NOTE: The [OpenEBS component](/docs/components/openebs/openebs.md) provides a
+>k8s-native persistent storage solution which can be used on almost any
+>infrastructure.
+
## Installation
#### `.lokocfg` file
