Fix: Update gatekeeper docs to include replicas example

General MR

Summary

Add an example of explicitly defining replicas for OPA gatekeeper for High availability. It is already set for replicas: 3 in the package by default, so I modified terminology to articulate that.

Looking at chart source (upstream) and docs, there is little mention of high-availability and any limitations to number of replicas.

Issues

Closes #1281 (closed)