install graphviz before just build

Author: CalMacCQ

.github/workflows/check-docs-build.yml

@@ -31,6 +31,10 @@ jobs:
       uses: actions/setup-python@v6
       with:
         python-version: 3.11
+    - name: install graphviz
+      run: |
+          sudo apt-get update
+          sudo apt-get install graphviz
     - name: build manual and examples
       run: |
         just build