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

examples/prometheus-additional-rendered-rule-example.jsonnet

@@ -1,12 +1,5 @@
 local kp = (import 'kube-prometheus/kube-prometheus.libsonnet') + {
-  _config+:: {
-    namespace: 'monitoring',
-    prometheus+:: {
-      renderedRules: {
-        'example.rules.yaml': (importstr 'example.rules.yaml'),
-      },
-    },
-  },
+  prometheusAlerts+:: (import 'existingrule.json'),
 };
 
 { ['00namespace-' + name]: kp.kubePrometheus[name] for name in std.objectFields(kp.kubePrometheus) } +