add custom domain cname attribute to publish

Author: mmcky

.github/workflows/publish.yml

@@ -32,4 +32,5 @@ jobs:
         uses: peaceiris/actions-gh-pages@v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: _build/html/
+          publish_dir: _build/html/
+          cname: python-programming.quantecon.org