Merge pull request #6194 from sbueringer/pr-update-book-refs-1.1

📖 book: update refs to older books

Author: k8s-ci-robot

docs/book/src/introduction.md

@@ -11,9 +11,17 @@ Started by the Kubernetes Special Interest Group (SIG) [Cluster Lifecycle](https
 * [Developer guide](./developer/guide.md)
 * [Contributing](./CONTRIBUTING.md)
 
-**Using Cluster API v1alpha3 or v1alpha4?** Please see the corresponding documentation:
-* [release-0-3.cluster-api.sigs.k8s.io](https://release-0-3.cluster-api.sigs.k8s.io)
+<aside class="note">
+
+<h1>ClusterAPI documentation versions</h1>
+
+This book documents ClusterAPI v1.1. For other Cluster API versions please see the corresponding documentation:
+* [main.cluster-api.sigs.k8s.io](https://main.cluster-api.sigs.k8s.io)
+* [release-1-0.cluster-api.sigs.k8s.io](https://release-1-0.cluster-api.sigs.k8s.io)
 * [release-0-4.cluster-api.sigs.k8s.io](https://release-0-4.cluster-api.sigs.k8s.io)
+* [release-0-3.cluster-api.sigs.k8s.io](https://release-0-3.cluster-api.sigs.k8s.io)
+
+</aside>
 
 ## Why build Cluster API?