Add docs

Author: jrauh01

doc/01-About.md

@@ -29,6 +29,8 @@ each connecting to a different cluster but writing data into the same database.
 The web interface accesses this database to display resource information and state,
 offering the flexibility to view aggregated data from all clusters or focus on a specific cluster.
 This setup ensures scalable monitoring and a unified view of resources across multiple Kubernetes environments.
+To monitor multiple clusters you have to 
+[run multiple service instances of Icinga for Kubernetes](03-Configuration.md#multiple-clusters).
 
 ## Vision and Roadmap
 

doc/03-Configuration.md

@@ -97,3 +97,43 @@ The configurations set by environment variables override the ones set by YAML.
 | PROMETHEUS_URL      | **Optional.** Prometheus server URL. If not set, metric synchronization is disabled. |
 | PROMETHEUS_USERNAME | **Optional.** Prometheus username.                                                   |
 | PROMETHEUS_PASSWORD | **Optional.** Prometheus password.                                                   |
+
+## Multiple Clusters
+
+### Systemd
+
+If you are running Icinga for Kubernetes with systemd, you have to follow these steps.
+
+#### Create Instance
+
+1. Create a new or edit the provided `default.env` file in `/etc/icinga-kubernetes`. 
+The file name will be the instance name for the systemd service. For example `test-cluster.env` will 
+start the service `icinga-kubernetes@test-cluster`.
+2. Set the `KUBECONFIG` env to configure how Icinga for Kubernetes can connect to the cluster.
+3. Set the `ICINGA_FOR_KUBERNETES_CLUSTER_NAME` env to configure the cluster name. If the env is not set
+the cluster name will be the .env file name.
+4. You can add additional envs to override the `config.yml` (look above to see more information about available envs).
+5. Reload the systemd daemon with `systemctl daemon-reload` to recognize the new cluster configs.
+6. Restart the Icinga for Kubernetes service with `systemctl restart icinga-kubernetes`.
+
+An example `test-cluster.env` file could look like this:
+```bash
+KUBECONFIG=/home/user/.kube/config
+ICINGA_FOR_KUBERNETES_CLUSTER_NAME="Test Cluster"
+ICINGA_FOR_KUBERNETES_PROMETHEUS_URL=http://localhost:9090
+```
+
+#### Remove Instance
+
+1. Remove the corresponding file from `/etc/icinga-kubernetes`.
+2. Reload the systemd daemon with `systemctl daemon-reload` to make sure the daemon forgets the file.
+3. Stop the service instance manually. For `test-cluster` it would be `systemctl stop icinga-kubernetes@test-cluster`.
+
+!!! Warning
+
+    If you stop the service without removing the env file, the instance will restart when the service is restarted.
+
+!!! Warning
+
+    If you remove the env file without stopping the instance, the instance will try to restart and 
+    fail when the service is restarted.