Fix RTD sphinx warning-as-error.

docs/conf.py

@@ -105,7 +105,7 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ['_static']
+html_static_path = []
 
 # Custom sidebar templates, must be a dictionary that maps document names
 # to template names.