kube-prometheus: Move service monitors to target folder

With 1d00e0ab ("ServiceMonitor: Enable Prometheus to select ServMon
outside own ns") merged: To emphasize the fact, that ServiceMonitors can
be deployed alongside their targets, this patch moves the Alertmanager,
Kube-State-Metrics, Node-Exporter and Prometheus Operator
Service-Monitor into the respective target folder in the kube-prometheus
project.

jsonnet/alertmanager/alertmanager-main-service-monitor.libsonnet

@@ -0,0 +1,32 @@
+{
+    new(namespace)::
+        {
+            "apiVersion": "monitoring.coreos.com/v1",
+            "kind": "ServiceMonitor",
+            "metadata": {
+                "name": "alertmanager",
+                "namespace": namespace,
+                "labels": {
+                    "k8s-app": "alertmanager"
+                }
+            },
+            "spec": {
+                "selector": {
+                    "matchLabels": {
+                        "alertmanager": "main"
+                    }
+                },
+                "namespaceSelector": {
+                    "matchNames": [
+                        "monitoring"
+                    ]
+                },
+                "endpoints": [
+                    {
+                        "port": "web",
+                        "interval": "30s"
+                    }
+                ]
+            }
+        }
+}