Add git diff so we can see what's wrong in CI

Author: Pierre-Sassoulas

.github/workflows/checks.yaml

@@ -182,5 +182,5 @@ jobs:
         run: |
           . venv/bin/activate
           cd doc
-          pre-commit run --hook-stage push sphinx-generated-doc --all-files || { echo "Make sure that there \
+          pre-commit run --hook-stage push sphinx-generated-doc --all-files || {git diff; echo "Make sure that there \
           are no modifications locally when launching 'make html'" ; exit 1; }