docs: add documentation and descriptor for using calico instead of flannel

docs/usage/guides/.pages

@@ -1,4 +1,5 @@
 title: Guides
 arrange:
   - exposing_services.md
-  - registries.md
+  - registries.md
+  - calico.md

docs/usage/guides/calico.md

@@ -0,0 +1,46 @@
+# Use Calico instead of Flannel
+If you want to use NetworkPolicy you can use Calico in k3s instead of Flannel.
+
+## 1. Disable Flannel
+On the k3s cluster creation add the flag `--flannel-backend=none`. For this on k3d you need to foward this flag to k3s with the option `--k3s-server-arg`.
+
+So the command of the cluster creation is
+```bash
+    k3d cluster create "${clustername}" --k3s-server-arg --flannel-backend=none
+```
+In this exemple change `"${clustername}"` with the name of the cluster (or set a variable). You can add other options, [see](../commands.md).  
+
+The cluster will start without flannel (and without other CNI).
+
+## 2. Install Calico
+Now you need to install Calico 
+
+### 2.1. Download and modify the Calico descriptor
+You can following the [documentation](https://docs.projectcalico.org/master/reference/cni-plugin/configuration)
+
+And then you have to change the ConfigMap `calico-config`. On the `cni_network_config` add the entry for allowing IP forwarding  
+```json
+    "container_settings": {
+        "allow_ip_forwarding": true
+    }
+```
+Or directly using the descriptor calico.yaml
+
+
+### 2.2. Apply the descriptor
+With kubectl (check the usage `kubectl kubeconfig` if you can't connect to the cluster).
+
+From the root of the repository
+```bash
+    kubectl apply -f docs/usage/guides/calico.yaml
+```
+
+And watch for the calico pod(s) deployment
+```
+    watch "kubectl get pods -n kube-system"
+```
+<!> Calico is not as quick as Flannel (but it provides more features)
+
+## References
+https://rancher.com/docs/k3s/latest/en/installation/network-options/  
+https://docs.projectcalico.org/getting-started/kubernetes/k3s/