docs: fix graphs on RTD

At OFC I was talking with the OpenROADM people, and it turned out that
our docs stopped rendering properly at RTD. It turned out that the build
started skipping the bindep.txt file at some (unknown) point in time.

Reported-by: Aparaajitha G L <aparaajitha.gomathinayakamlatha@utdallas.edu>
Change-Id: Ie9a4b61f36fb979fb5c109d02de06e0b2cbf270e

.readthedocs.yml

@@ -3,6 +3,8 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3.12"
+  apt_packages:
+    - graphviz
 
 python:
   install: