CI: turn warnings into errors when building the documentation

Make it easier to catch errors in the documentation.

Author: dnicolodi

.github/workflows/docs.yml

@@ -33,7 +33,7 @@ jobs:
         with:
           ref: ${{ github.event.push.tag || github.event.inputs.tag }}
       - run: python -m pip install .[docs]
-      - run: python -m sphinx docs/ build/docs/
+      - run: python -m sphinx -W docs/ build/docs/
       - uses: actions/upload-pages-artifact@v2
         with:
           path: build/docs/