feature: Add manifest fetch documentation for digest (#1465)

Signed-off-by: Terry Howe <[email protected]>

cmd/oras/root/manifest/fetch.go

@@ -55,6 +55,9 @@ Example - Fetch raw manifest from a registry:
 Example - Fetch the descriptor of a manifest from a registry:
   oras manifest fetch --descriptor localhost:5000/hello:v1
 
+Example - Fetch the manifest digest from a registry:
+  oras manifest fetch --format go-template --template '{{ .digest }}' localhost:5000/hello:v1
+
 Example - Fetch manifest from a registry with specified media type:
   oras manifest fetch --media-type 'application/vnd.oci.image.manifest.v1+json' localhost:5000/hello:v1