Trying to get readthedocs index back (#808)

Merging random changes hoping to hit on the one that's breaking the docs.  Surely there's a better way?  :-(

.readthedocs.yaml

@@ -11,10 +11,12 @@ build:
   apt_packages:
     - doxygen
     - graphviz
+    - python3-graphviz
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
-  #builder: html
+  # builder: html
+  builder: dirhtml
   configuration: doc/conf.py
   fail_on_warning: true
 
@@ -24,3 +26,5 @@ sphinx:
 #   install:
 #   - requirements: docs/requirements.txt
 
+sphinx:
+  configuration: doc/conf.py