Add docker-registry secrets to OCI doc

A new statement is added to specify that docker-registry secrets are
accepted by OCI Helm repositories.

Signed-off-by: Soule BA <[email protected]>

Author: souleb

docs/spec/v1beta2/helmrepositories.md

@@ -240,7 +240,7 @@ metadata:
   namespace: default
 spec:
   interval: 5m0s
-  url: oci://ghcr.io/stefanprodan/charts
+  url: oci://ghcr.io/my-user/my-private-repo
   type: "oci"
   secretRef:
     name: oci-creds
@@ -255,6 +255,8 @@ stringData:
   password: 123456
 ```
 
+For OCI Helm repositories, it is also possible to use a kubernetes secret of type `docker-registry`.
+
 #### TLS authentication
 
 **Note:** TLS authentication is not yet supported by OCI Helm repositories.