Fix diagrams by reinstating graphiz

.readthedocs.yaml

@@ -1,16 +1,18 b''
 version: 2
 
 build:
   os: ubuntu-22.04
   tools:
     python: "3.11"
+  apt_packages:
+    - graphviz
 
 sphinx:
   configuration: docs/source/conf.py
 
 # Optional but recommended, declare the Python requirements required
 # to build your documentation
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
    install:
    - requirements: docs/requirements.txt