ci: install graphviz for docs build

.github/workflows/docs.yml

@@ -55,6 +55,10 @@ jobs:
  with:
  python-version: '3.11'
  cache-dependency-path: docs/requirements.txt
+ - name: Install apt dependencies
+ run: |
+ sudo apt update
+ sudo apt install -y graphviz
  - name: Install Python dependencies
  run: |
  pip install uv