chore: set generate_blocked to true for documentai-toolbox (#16134)

This PR updates `.librarian/config.yaml` to set `generate_blocked` to
true for `google-cloud-documentai-toolbox`. This will unblock generate
for `google-cloud-documentai-toolbox`.

Author: ohmayr

.librarian/config.yaml

@@ -9,3 +9,4 @@ libraries:
 # explicitly initiated
   - id: "google-cloud-documentai-toolbox"
     release_blocked: true
+    generate_blocked: true