CI/check-docs: use explicit Python version

Use explicit Python version to match the Python version used in
.readthedocs.yml since the different Python version leads to different
versions of sphinx dependencies that can change the auto-generated docs.

.github/workflows/ci.yaml

@@ -346,6 +346,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/setup-python@v5
+        with:
+          # This should match the python version in .readthedocs.yml
+          python-version: '3.11'
 
       - uses: actions/checkout@v4