added Pandoc

Author: mbarber

.github/workflows/s3.yaml

@@ -20,7 +20,7 @@ jobs:
     - name: Install Sphinx
       run: |
         python -m pip install --upgrade pip
-        pip install sphinx recommonmark sphinx_markdown_tables sphinx_markdown_builder nbsphinx sphinx-rtd-theme
+        pip install sphinx recommonmark sphinx_markdown_tables sphinx_markdown_builder nbsphinx sphinx-rtd-theme pandoc
 
     - name: Build HTML with Sphinx
       run: |