Add sphinx.ext.autodoc for doc build

After switching packaging to use build_sphinx, we need to add this
extension to avoid errors like:

 ERROR: Unknown directive type "automodule".

during doc build.

Change-Id: Ia9997b466d31a9422de64549a14196af173b1f63

docs/conf.py

@@ -41,7 +41,7 @@ sys.path.append('..')
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = []
+extensions = ['sphinx.ext.autodoc']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']