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

@@ -368,7 +368,7 @@ 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
@@ -383,6 +383,16 @@ stringData:
   password: 123456
 ```
 
+For OCI Helm repositories, Kubernetes secrets of type `docker-registry` are also supported.
+It is possible to create one such secret with the following fields:
+
+```yaml
+flux create secret oci ghcr-auth \
+  --url=ghcr.io \
+  --username=flux \
+  --password=${GITHUB_PAT}
+```
+
 #### TLS authentication
 
 **Note:** TLS authentication is not yet supported by OCI Helm repositories.