Install graphviz in CI

To generate the example that has graphviz, the `dot` utility needs to be
available, which is not by default.  This commit makes the graphviz
application available in the CI services that build the example.

.github/workflows/CI.yml

@@ -145,6 +145,19 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
           pandoc: true
+      - name: "Install Graphviz on Windows 🗔"
+        if: runner.os == 'Windows'
+        run: choco install graphviz
+      - name: "Install Graphviz on macOS "
+        if: runner.os == 'macOS'
+        run: |
+          brew update
+          brew install graphviz
+      - name: "Install Graphviz on Linux 🐧"
+        if: runner.os == 'Linux'
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends graphviz
       - name: "Build docs and check for warnings 📖"
         shell: bash
         run: |

readthedocs.yml

@@ -7,6 +7,8 @@ build:
   os: ubuntu-20.04
   tools:
     python: "3.10"
+  apt_packages:
+    - graphviz
   jobs:
     # build the gallery of themes before building the doc
     post_install: