book: Add API reference page

Author: sbueringer

docs/book/src/SUMMARY.md

@@ -74,6 +74,7 @@
     - [CustomResourceDefinitions relationships](./developer/crd-relationships.md)
 - [Troubleshooting](./user/troubleshooting.md)
 - [Reference](./reference/reference.md)
+    - [API Reference](./reference/api_reference.md) 
     - [Glossary](./reference/glossary.md)
     - [Provider List](./reference/providers.md)
     - [Ports](./reference/ports.md)

docs/book/src/reference/api_reference.md

@@ -0,0 +1,7 @@
+# API Reference
+
+Cluster API currently exposes the following APIs:
+
+* the Cluster API Custom Resource Definitions (CRDs): [documentation](https://doc.crds.dev/github.com/kubernetes-sigs/cluster-api)
+
+* Golang APIs: [godoc](https://pkg.go.dev/sigs.k8s.io/cluster-api)