Add release 1.2 book link

Author: Yuvaraj Kakaraparthi

docs/book/src/introduction.md

@@ -15,8 +15,9 @@ Started by the Kubernetes Special Interest Group (SIG) [Cluster Lifecycle](https
 
 <h1>ClusterAPI documentation versions</h1>
 
-This book documents ClusterAPI v1.2. For other Cluster API versions please see the corresponding documentation:
+This book documents ClusterAPI v1.3. For other Cluster API versions please see the corresponding documentation:
 * [main.cluster-api.sigs.k8s.io](https://main.cluster-api.sigs.k8s.io)
+* [release-1-2.cluster-api.sigs.k8s.io](https://release-1-2.cluster-api.sigs.k8s.io)
 * [release-1-1.cluster-api.sigs.k8s.io](https://release-1-1.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)