Add graphviz package to doc build config (cocotb#3457)

Add graphviz package to doc build config, directly use SVG

This makes the 'dot' binary available to sphinx.ext.inheritance-diagrams.
See also readthedocs/readthedocs.org#8672.
Include rendered SVG directly until sphinx-doc/sphinx#10117 is fixed.

Closes cocotb#3449

Author: cmarqu

.readthedocs.yml

@@ -8,6 +8,8 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3.11"
+  apt_packages:
+    - graphviz
 
 python:
   install:

documentation/source/diagrams/svg/inheritance-cocotb.handle.svg

@@ -188,8 +188,8 @@ Logging
 Simulation Object Handles
 =========================
 
-.. inheritance-diagram:: cocotb.handle
-   :parts: 1
+.. image:: diagrams/svg/inheritance-cocotb.handle.svg
+   :alt: The class inheritance diagram for cocotb.handle
 
 .. currentmodule:: cocotb.handle