install graphviz when building docs for gh-pages

Author: ericniebler

.github/workflows/gh-pages.yml

@@ -34,7 +34,7 @@ jobs:
 
     - name: Install dependencies
       run: |
-        sudo apt-get install -y doxygen python3-sphinx python3-sphinx-rtd-theme
+        sudo apt-get install -y doxygen python3-sphinx python3-sphinx-rtd-theme graphviz
         pip3 install breathe sphinxcontrib-bibtex sphinx_copybutton IPython myst_parser \
           nbsphinx