docs: Add breaking changes section to introduction.md

We want a way to call out some upcoming breaking changes for new
versions. This adds a note style callout in the
`ClusterAPI documentation versions` section.

docs/book/src/introduction.md

@@ -25,6 +25,14 @@ This book documents ClusterAPI v1.4. For other Cluster API versions please see t
 * [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)
 
+> ⚠️ Break Changes ⚠️
+>
+> API Version v1alpha3 support will be removed with the upcoming release of v1.5
+>
+> API Version v1alpha4 support will be removed with the upcoming release of v1.6
+>
+> Review the [CONTRIBUTING support-and-guarantees](https://github.com/kubernetes-sigs/cluster-api/blob/main/CONTRIBUTING.md#support-and-guarantees) for more details
+
 </aside>
 
 ## Why build Cluster API?