Install graphviz. Add note to docs (#234)

Author: kinow

.github/workflows/gh-pages.yaml

@@ -6,13 +6,19 @@ on:
       - main
   workflow_dispatch:
 
+# NOTE: If you modify this file to install a package with pip or apt, please
+#       verify if we need the same package added to our readthedocs build.
 jobs:
   # Ref: https://stackoverflow.com/questions/57989790/using-github-actions-to-publish-documentation
   deploy:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
 
+      - name: Install apt packages
+        run: |
+          sudo apt-get install -y graphviz
+
       - name: Set up Python
         uses: actions/setup-python@v4
         with:

.readthedocs.yaml

@@ -1,5 +1,7 @@
 version: 2
 
+# NOTE: If you modify this file to install a package with pip or apt, please
+#       verify if we need the same package added to our CI.
 build:
   os: ubuntu-20.04
   tools:

CONTRIBUTING.md

@@ -111,6 +111,13 @@ source venv/bin/activate
 # Open <http://localhost:8000/> in your browser
 ```
 
+> NOTE: When you modify the packages installed with apt or pip, please verify
+> if the change needs to be applied to either or to both of CI and readthedocs.
+> ReadTheDocs builds and deploys previews. CI builds and deploys production
+> (GitHub Actions). Failing to update both may result in previews generated
+> correctly, but failure to deploy the production version after the pull request
+> gets merged.
+
 ## Other Resources
 
 General discussion of [Common Workflow Language][cwl-site] project