GitHub Actions: Don't build with warnings-as-errors for offline doc

The class reference may have invalid parts which otherwise prevent
building the offline documentation.

.github/workflows/build_offline_docs.yml

@@ -28,7 +28,7 @@ jobs:
       # Build the HTML to upload it.
       - name: Sphinx build
         run: |
-          sphinx-build --color -d _build/doctrees -W . _build/html
+          sphinx-build --color -d _build/doctrees . _build/html
 
       - uses: actions/upload-artifact@v2
         with: