install build dependencies during action

.github/workflows/docs.yml

@@ -37,6 +37,8 @@ jobs:
           path: "./docs"  # Upload entire repository
       - name: Install package
         run: python -m pip install .
+      - name: Install dependencies
+        run: python -m pip install sphinx pydata-sphinx-theme
       - name: Deploy to GitHub Pages
         id: deployment
         uses: sphinx-notes/pages@v3