add pandoc to docs.yaml

.github/workflows/docs.yaml

@@ -11,6 +11,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v3
+      - uses: docker://pandoc/core:2.9
       - name: Install dependencies
         run: |
           pip install sphinx sphinx_rtd_theme nbsphinx