Update docs for ClusterResourceSets

The current documentation links to the original enhancement proposal for
examples, but these examples are for an older API version. This commit
adds examples directly in the book for the latest API version.

Signed-off-by: Lennart Jern <lennart.jern@est.tech>

docs/book/src/tasks/experimental-features/cluster-resource-set.md

@@ -8,9 +8,44 @@ The `ClusterResourceSet` feature is introduced to provide a way to automatically
 
 The `ClusterResourceSet` feature is enabled by default, but can be disabled by setting the `EXP_CLUSTER_RESOURCE_SET` environment variable to `false`.
 
-More details on `ClusterResourceSet` and an example to test it can be found at:
+More details on `ClusterResourceSet` can be found at:
 [ClusterResourceSet CAEP](https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20200220-cluster-resource-set.md)
 
+## Example
+
+Suppose you want to automatically install the relevant external cloud provider on all workload clusters.
+This can be accomplished by labeling the clusters with the specific cloud (e.g. AWS, GCP or OpenStack) and then creating a `ClusterResourceSet` for each.
+For example, you could have the following for OpenStack:
+
+```yaml
+apiVersion: addons.cluster.x-k8s.io/v1beta1
+kind: ClusterResourceSet
+metadata:
+  name: cloud-provider-openstack
+  namespace: default
+spec:
+  strategy: Reconcile
+  clusterSelector:
+    matchLabels:
+      cloud: openstack
+  resources:
+    - name: cloud-provider-openstack
+      kind: ConfigMap
+    - name: cloud-config
+      kind: Secret
+```
+
+This `ClusterResourceSet` would apply the content of the `Secret` `cloud-config` and of the `ConfigMap` `cloud-provider-openstack` in all workload clusters with the label `cloud=openstack`.
+Suppose you have the file `cloud.conf` that should be included in the `Secret` and `cloud-provider-openstack.yaml` that should be in the `ConfigMap`.
+The `Secret` and `ConfigMap` can then be created in the following way:
+
+```bash
+kubectl create secret generic cloud-config --from-file=cloud.conf --type=addons.cluster.x-k8s.io/resource-set
+kubectl create configmap cloud-provider-openstack --from-file=cloud-provider-openstack.yaml
+```
+
+Note that it is required that the `Secret` has the type `addons.cluster.x-k8s.io/resource-set` for it to be picked up.
+
 ## Update from `ApplyOnce` to `Reconcile`
 
 The `strategy` field is immutable so existing CRS can't be updated directly. However, CAPI won't delete the managed resources in the target cluster when the CRS is deleted.