Add drop dashboards example

Signed-off-by: ArthurSens <arthursens2005@gmail.com>

docs/customizations/dropping-unwanted-dashboards.md

+### Dropping unwanted dashboards
+
+When deploying kube-prometheus, your Grafana instance is deployed with a lot of dashboards by default. All those dashboards are comming from upstream projects like [kubernetes-mixin](https://github.com/kubernetes-monitoring/kubernetes-mixin), [prometheus-mixin](https://github.com/prometheus/prometheus/tree/main/documentation/prometheus-mixin) and [node-exporter-mixin](https://github.com/prometheus/node_exporter/tree/master/docs/node-mixin), among others.
+
+In case you find out that you don't need some of them, you can choose to remove those dashboards like in the example below, which removes the [`alertmanager-overview.json`](https://github.com/prometheus/alertmanager/blob/main/doc/alertmanager-mixin/dashboards/overview.libsonnet) dashboard.
+
+```jsonnet mdox-exec="cat examples/drop-dashboards.jsonnet"
+local kp =
+  (import 'kube-prometheus/main.libsonnet') +
+  {
+    values+:: {
+      common+: {
+        namespace: 'monitoring',
+      },
+      grafana+: {
+        dashboards: std.mergePatch(super.dashboards, {
+          // Add more unwanted dashboards here
+          'alertmanager-overview.json': null,
+        }),
+      },
+    },
+  };
+
+{ 'setup/0namespace-namespace': kp.kubePrometheus.namespace } +
+{
+  ['setup/prometheus-operator-' + name]: kp.prometheusOperator[name]
+  for name in std.filter((function(name) name != 'serviceMonitor' && name != 'prometheusRule'), std.objectFields(kp.prometheusOperator))
+} +
+// serviceMonitor and prometheusRule are separated so that they can be created after the CRDs are ready
+{ 'prometheus-operator-serviceMonitor': kp.prometheusOperator.serviceMonitor } +
+{ 'prometheus-operator-prometheusRule': kp.prometheusOperator.prometheusRule } +
+{ 'kube-prometheus-prometheusRule': kp.kubePrometheus.prometheusRule } +
+{ ['alertmanager-' + name]: kp.alertmanager[name] for name in std.objectFields(kp.alertmanager) } +
+{ ['blackbox-exporter-' + name]: kp.blackboxExporter[name] for name in std.objectFields(kp.blackboxExporter) } +
+{ ['grafana-' + name]: kp.grafana[name] for name in std.objectFields(kp.grafana) } +
+{ ['kube-state-metrics-' + name]: kp.kubeStateMetrics[name] for name in std.objectFields(kp.kubeStateMetrics) } +
+{ ['kubernetes-' + name]: kp.kubernetesControlPlane[name] for name in std.objectFields(kp.kubernetesControlPlane) }
+{ ['node-exporter-' + name]: kp.nodeExporter[name] for name in std.objectFields(kp.nodeExporter) } +
+{ ['prometheus-' + name]: kp.prometheus[name] for name in std.objectFields(kp.prometheus) } +
+{ ['prometheus-adapter-' + name]: kp.prometheusAdapter[name] for name in std.objectFields(kp.prometheusAdapter) }
+```

examples/drop-dashboards.jsonnet

+local kp =
+  (import 'kube-prometheus/main.libsonnet') +
+  {
+    values+:: {
+      common+: {
+        namespace: 'monitoring',
+      },
+      grafana+: {
+        dashboards: std.mergePatch(super.dashboards, {
+          // Add more unwanted dashboards here
+          'alertmanager-overview.json': null,
+        }),
+      },
+    },
+  };
+
+{ 'setup/0namespace-namespace': kp.kubePrometheus.namespace } +
+{
+  ['setup/prometheus-operator-' + name]: kp.prometheusOperator[name]
+  for name in std.filter((function(name) name != 'serviceMonitor' && name != 'prometheusRule'), std.objectFields(kp.prometheusOperator))
+} +
+// serviceMonitor and prometheusRule are separated so that they can be created after the CRDs are ready
+{ 'prometheus-operator-serviceMonitor': kp.prometheusOperator.serviceMonitor } +
+{ 'prometheus-operator-prometheusRule': kp.prometheusOperator.prometheusRule } +
+{ 'kube-prometheus-prometheusRule': kp.kubePrometheus.prometheusRule } +
+{ ['alertmanager-' + name]: kp.alertmanager[name] for name in std.objectFields(kp.alertmanager) } +
+{ ['blackbox-exporter-' + name]: kp.blackboxExporter[name] for name in std.objectFields(kp.blackboxExporter) } +
+{ ['grafana-' + name]: kp.grafana[name] for name in std.objectFields(kp.grafana) } +
+{ ['kube-state-metrics-' + name]: kp.kubeStateMetrics[name] for name in std.objectFields(kp.kubeStateMetrics) } +
+{ ['kubernetes-' + name]: kp.kubernetesControlPlane[name] for name in std.objectFields(kp.kubernetesControlPlane) }
+{ ['node-exporter-' + name]: kp.nodeExporter[name] for name in std.objectFields(kp.nodeExporter) } +
+{ ['prometheus-' + name]: kp.prometheus[name] for name in std.objectFields(kp.prometheus) } +
+{ ['prometheus-adapter-' + name]: kp.prometheusAdapter[name] for name in std.objectFields(kp.prometheusAdapter) }