RoutablePageMixin has a default index_route method that is decorated
with `@route(r'^$')`. This way, including RoutablePageMixin doesn't
force you to re-enable the default functionality you would expect from a
Page anyways.
index_route behaves exactly like a standard Wagtail Page.
To override the default behaviour, one can simply override the route by
decorating another function with r'^$'.
(as disussed in issue #2866)
