L2SS-698: Install graphviz for documentation

.gitlab-ci.yml

@@ -442,7 +442,7 @@ sphinx-documentation:
   stage: documentation
   before_script:
     - sudo apt-get update
-    - sudo apt-get install -y git
+    - sudo apt-get install -y git graphviz
   script:
     - cd tangostationcontrol
     - tox -e docs