Add submodule configuration to rtd yaml (#1007)

.readthedocs.yml

@@ -5,6 +5,10 @@
 # Required
 version: 2
 
+submodules:
+  include: all
+  recursive: true
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/source/conf.py
@@ -13,7 +17,6 @@ sphinx:
 # Optionally build your docs in additional formats such as PDF and ePub
 formats:
   - htmlzip
-  - pdf
 
 # Optionally set the version of Python and requirements required to build your docs
 python: