Doc: Add sphinx.ext.graphviz extension dependency

Author: adorilson

Doc/conf.py

@@ -38,6 +38,7 @@
     'sphinx.ext.coverage',
     'sphinx.ext.doctest',
     'sphinx.ext.extlinks',
+    'sphinx.ext.graphviz',
 ]
 
 # Skip if downstream redistributors haven't installed them
@@ -560,6 +561,10 @@
 }
 extlinks_detect_hardcoded_links = True
 
+# Options for sphinx.ext.graphviz
+# -------------------------------
+graphviz_output_format = 'svg'
+
 # Options for c_annotations extension
 # -----------------------------------