add sphinx_rtd_theme to sphinx workflow

Author: alperaltuntas

.github/workflows/sphinx.yml

@@ -11,6 +11,8 @@ jobs:
     - uses: actions/checkout@v4
     - name: Build HTML
       uses: ammaraskar/sphinx-action@master
+      with:
+        pre-build-command: python -m pip install sphinx_rtd_theme
     - name: Upload artifacts
       uses: actions/upload-artifact@v4
       with: