Docs: add very simple architecture diagram

Change-Id: Ib0552cafd80c8a602c984cf6ca9f7f4ca8e687fe
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 1120bfc..76ee6bc 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -27,6 +27,7 @@
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = [
     'sphinx.ext.autodoc',
+    'sphinx.ext.graphviz',
     'sphinxcontrib.blockdiag',
     'sphinxcontrib.programoutput',
     'zuul.sphinx.zuul',