Add back pandoc install

Author: rafa-guedes

.github/workflows/sphinx_docs_to_gh_pages.yaml

@@ -17,6 +17,7 @@ jobs:
           python-version: "3.10"
       - name: Install dependencies
         run: |
+          sudo apt install pandoc
           python -m pip install --upgrade pip
           pip install .
           pip install -r docs/requirements.txt