Update publish_documentation.yml

So that after PR is submitted the doc is built and updated.

.github/workflows/publish_documentation.yml

@@ -19,6 +19,17 @@ jobs:
         uses: actions/[email protected]
         with:
           persist-credentials: false
+
+      - name: Set up Python 3.9
+        uses: actions/setup-python@v1
+        with:
+          python-version: 3.9
+
+      - name: Build docs
+        run: |
+          pip install -r requirements-doc.txt
+          sphinx-build -b html docs docs/build/html
+
       - name: Publish the documentation
         if: success() && github.ref == 'refs/heads/main'
         uses: JamesIves/[email protected]