Configure Renovate to keep Python at 3.10 for docs workflow

This change adds a packageRule to `renovate.json` to prevent Renovate
from updating the Python version used in the `.github/workflows/docs.yml`
GitHub Actions workflow.

The rule specifically targets the `python-version` input of the
`actions/setup-python` step and restricts allowed versions to `<3.11`,
effectively pinning it to `3.10` for now.

Author: google-labs-jules[bot]

renovate.json

@@ -8,5 +8,12 @@
   "ignorePaths": [".pre-commit-config.yaml", ".kokoro/requirements.txt", "setup.py", ".github/workflows/unittest.yml"],
   "pip_requirements": {
     "fileMatch": ["requirements-test.txt", "samples/[\\S/]*constraints.txt", "samples/[\\S/]*constraints-test.txt"]
-  }
+  },
+  "packageRules": [
+    {
+      "matchFileNames": [".github/workflows/docs.yml"],
+      "matchStrings": ["uses: actions/setup-python@.*\\n          with:\\n            python-version: '(.*)'"],
+      "allowedVersions": "<3.11"
+    }
+  ]
 }