[Sphinx documentation] Set master_doc to "index"

docs/finn/conf.py

@@ -50,3 +50,5 @@ html_theme = 'alabaster'
 # 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']
+
+master_doc = 'index'