doc: add capi how-to for custom bootstrap config (#1122)

We're adding a how-to document, showing how to use a custom
bootstrap configuration with the Cluster API provider.

Note that this is already outlined by the config reference page:
https://documentation.ubuntu.com/canonical-kubernetes/latest/capi/reference/configs/#bootstrapconfig

docs/canonicalk8s/capi/howto/custom-bootstrap-config.md

@@ -0,0 +1,107 @@
+# How to use custom bootstrap configuration
+
+The {{product}} bootstrap configuration gets automatically generated based on
+user provided settings described in the [Cluster API configuration reference].
+
+The configuration generated by the CAPI provider will also include CA
+certificates as well as annotations and other settings that allow the provider
+to function properly.
+
+Not all bootstrap options are exposed through CAPI settings. However,
+users can explicitly define the {{product}} bootstrap configuration.
+This completely bypasses the other CAPI provider settings and the configuration
+will be passed as-is to the {{product}} snap.
+
+See the [Bootstrap configuration file reference] for more details about the
+available settings.
+
+## Passing the bootstrap configuration directly
+
+The bootstrap configuration can be specified in the ``CK8sControlPlane`` spec:
+
+```
+apiVersion: controlplane.cluster.x-k8s.io/v1beta2
+kind: CK8sControlPlane
+metadata:
+  name: c1-control-plane
+  namespace: default
+spec:
+  machineTemplate:
+    infrastructureTemplate:
+      apiVersion: infrastructure.cluster.x-k8s.io/v1beta1
+      kind: DockerMachineTemplate
+      name: c1-control-plane
+  replicas: 1
+  spec:
+    bootstrapConfig:
+      content: |
+        cluster-config:
+          annotations:
+            k8sd/v1alpha/lifecycle/skip-cleanup-kubernetes-node-on-remove: "true"
+            k8sd/v1alpha/lifecycle/skip-stop-services-on-remove: "true"
+          network:
+            enabled: true
+          dns:
+            enabled: true
+          local-storage:
+            enabled: true
+            reclaim-policy: Retain
+```
+
+Note that the k8sd annotations allow the CAPI provider to properly remove
+nodes.
+
+## Using secrets to store the bootstrap configuration
+
+The bootstrap configuration may contain sensitive data. For this reason, the
+provider also allows passing it as a secret.
+
+```
+apiVersion: v1
+kind: Secret
+metadata:
+  name: ck8s-bootstrap-config
+type: Opaque
+stringData:
+  content: |
+    cluster-config:
+      annotations:
+        k8sd/v1alpha/lifecycle/skip-cleanup-kubernetes-node-on-remove: "true"
+        k8sd/v1alpha/lifecycle/skip-stop-services-on-remove: "true"
+      network:
+        enabled: true
+      dns:
+        enabled: true
+      local-storage:
+        enabled: true
+        reclaim-policy: Retain
+```
+
+The secret can then be referenced like so:
+
+```
+apiVersion: controlplane.cluster.x-k8s.io/v1beta2
+kind: CK8sControlPlane
+metadata:
+  name: c1-control-plane
+  namespace: default
+spec:
+  machineTemplate:
+    infrastructureTemplate:
+      apiVersion: infrastructure.cluster.x-k8s.io/v1beta1
+      kind: DockerMachineTemplate
+      name: c1-control-plane
+  replicas: 1
+  spec:
+    bootstrapConfig:
+      contentFrom:
+        secret:
+          # Name of the secret in the CK8sBootstrapConfig's namespace to use.
+          name: ck8s-bootstrap-config
+          # The key in the secret's data map for this value.
+          key: content
+```
+
+<!-- LINKS -->
+[Cluster API configuration reference]: /capi/reference/configs.md
+[Bootstrap configuration file reference]: /snap/reference/config-files/bootstrap-config.md

docs/canonicalk8s/capi/howto/index.md

@@ -17,6 +17,7 @@ Overview <self>
 :titlesonly:
 Provision a Canonical Kubernetes cluster <provision>
 Install custom Canonical Kubernetes <custom-ck8s>
+Use custom bootstrap configuration <custom-bootstrap-config>
 ```
 
 ## Upgrade