CHG: Add graphviz to rdt build

- apparently now one has to specifically install those packages
readthedocs/readthedocs.org#8800

Changes to be committed:
	modified:   .readthedocs.yml

Author: tensionhead

.readthedocs.yml

@@ -13,6 +13,8 @@ build:
   os: ubuntu-22.04
   tools:
     python: '3.10'
+  apt_packages:
+    - graphviz
   jobs:
     post_install:
       - pip install poetry