use theme provided by pallets-sphinx-themes

docs/tutorial/index.rst

@@ -12,10 +12,7 @@ as a database (which comes out of the box with Python) so there is nothing
 else you need.
 
 If you want the full source code in advance or for comparison, check out
-the `example source`_.
-
-.. _example source:
-   https://github.com/pallets/flask/tree/master/examples/flaskr/
+the :gh:`example source <examples/flaskr>`.
 
 .. toctree::
    :maxdepth: 2