fix readthedocs (#1864)

docs/conf.py

@@ -49,6 +49,9 @@ source_suffix = {
     '.md': 'markdown',
 }
 
+# The master toctree document.
+master_doc = 'index'
+
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
 # This pattern also affects html_static_path and html_extra_path.