Merge pull request kubernetes-sigs#9767 from mjlshen/cluster-api-release-1.6-book

📖 add release 1.5 book link

Author: k8s-ci-robot

docs/book/src/introduction.md

@@ -40,8 +40,9 @@ contributing guide for more details.
 
 <h1>ClusterAPI documentation versions</h1>
 
-This book documents ClusterAPI v1.5. For other Cluster API versions please see the corresponding documentation:
+This book documents ClusterAPI v1.6. For other Cluster API versions please see the corresponding documentation:
 * [main.cluster-api.sigs.k8s.io](https://main.cluster-api.sigs.k8s.io)
+* [release-1-5.cluster-api.sigs.k8s.io](https://release-1-5.cluster-api.sigs.k8s.io)
 * [release-1-4.cluster-api.sigs.k8s.io](https://release-1-4.cluster-api.sigs.k8s.io)
 * [release-1-3.cluster-api.sigs.k8s.io](https://release-1-3.cluster-api.sigs.k8s.io)
 * [release-1-2.cluster-api.sigs.k8s.io](https://release-1-2.cluster-api.sigs.k8s.io)