chore: add documentai-toolbox to config.yaml (#16133)

Unblock automated release by adding `google-cloud-documentai-toolbox` to
`.librarian/config.yaml`.

Author: ohmayr

.librarian/config.yaml

@@ -4,3 +4,8 @@ global_files_allowlist:
   # versions which are hardcoded in the file.
   - path: "CHANGELOG.md"
     permissions: "read-write"
+libraries:
+# libraries have "release_blocked: true" so that releases are
+# explicitly initiated
+  - id: "google-cloud-documentai-toolbox"
+    release_blocked: true