enh: install graphviz for doc generation

Author: sanjayankur31

.github/workflows/master.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Install deps
       run: |
         sudo apt-get update -qq
-        sudo apt-get install libxerces-c-dev xsdcxx cmake doxygen -qq
+        sudo apt-get install libxerces-c-dev xsdcxx cmake doxygen graphviz -qq
         python -m pip install --upgrade pip
         pip install ghp-import