docs: install pandoc and graphviz before building

.readthedocs.yaml

@@ -13,6 +13,7 @@ build:
   jobs:
     post_install:
       - pip install '.[dev]'
+      - apt update && apt install -y pandoc graphviz
     # You can also specify other tool versions:
     # nodejs: "19"
     # rust: "1.64"