.readthedocs.yaml: fix Read the Docs build issue

fix container, Python version, path to sphinx' conf.py and add all
required packages

.readthedocs.yaml

@@ -3,6 +3,16 @@ version: 2
 submodules:
   exclude: all
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.10"
+  apt_packages:
+    - graphviz 
+
+sphinx:
+  configuration: doc/conf.py
+
 python:
    install:
    - requirements: doc/requirements.txt

doc/requirements.txt

 breathe
+sphinx-rtd-theme