kube-prometheus/docs: Update how to handle existing rules

examples/existingrule.json

@@ -0,0 +1 @@
+{"groups":[{"name":"example-group","rules":[{"alert":"DeadMansSwitch","annotations":{"description":"This is a DeadMansSwitch meant to ensure that the entire alerting pipeline is functional."},"expr":"vector(1)","labels":{"severity":"none"}}]}]}