Merge pull request #12074 from chandankumar4/cluster-api-release-1.10-book

📖 add release 1.9 book link

Author: k8s-ci-robot

docs/book/src/introduction.md

@@ -16,8 +16,9 @@ Started by the Kubernetes Special Interest Group (SIG) [Cluster Lifecycle](https
 
 <h1>ClusterAPI documentation versions</h1>
 
-This book documents ClusterAPI v1.9. For other Cluster API versions please see the corresponding documentation:
+This book documents ClusterAPI v1.10. For other Cluster API versions please see the corresponding documentation:
 * [main.cluster-api.sigs.k8s.io](https://main.cluster-api.sigs.k8s.io)
+* [release-1-9.cluster-api.sigs.k8s.io](https://release-1-9.cluster-api.sigs.k8s.io)
 * [release-1-8.cluster-api.sigs.k8s.io](https://release-1-8.cluster-api.sigs.k8s.io)
 * [release-1-7.cluster-api.sigs.k8s.io](https://release-1-7.cluster-api.sigs.k8s.io)
 * [release-1-6.cluster-api.sigs.k8s.io](https://release-1-6.cluster-api.sigs.k8s.io)