Docs: Add graphviz to system requirements of RTD build runner (#6004)

The `graphviz` package provides the `dot` executable which is needed to
generate provenance graph. Without this, the tutorials that produce
provenance graph representations will raise an exception.

.readthedocs.yml

@@ -7,6 +7,8 @@ version: 2
 formats: []
 
 build:
+  apt_packages:
+    - graphviz
   os: ubuntu-22.04
   tools:
     python: "3.10"