Add link to Operator docs for RU (#1102)

If you deployed using Operator, the RU steps differ - added short explanation and link to Operator docs to highlight this difference.

docs/modules/maintain-cluster/pages/rolling-upgrades.adoc

@@ -53,6 +53,10 @@ NOTE: Following the rolling upgrade, you must resubmit jobs as they don't run du
 [[rolling-upgrade-procedure]]
 == Performing a Rolling Upgrade
 
+If you deployed your cluster using the Hazelcast Operator, follow the steps in the link:https://docs.hazelcast.com/operator/latest/scaling-upgrading#upgrading[Operator documentation, window=_blank].
+
+To upgrade your cluster after deploying with Hazelcast Platform, complete the following steps:
+
 . Gracefully shut down an existing member.
 +
 See xref:shutdown.adoc#shutting-down-a-hazelcast-member[Shutting Down a Hazelcast Member].