Fixed #28593 -- Added a simplified URL routing syntax per DEP 0201.

Thanks Aymeric Augustin for shepherding the DEP and patch review.
Thanks Marten Kenbeek and Tim Graham for contributing to the code.
Thanks Tom Christie, Shai Berger, and Tim Graham for the docs.

tests/check_framework/urls/beginning_with_slash.py

@@ -1,5 +1,7 @@
 from django.conf.urls import url
+from django.urls import path
 
 urlpatterns = [
-    url(r'/starting-with-slash/$', lambda x: x),
+    path('/path-starting-with-slash/', lambda x: x),
+    url(r'/url-starting-with-slash/$', lambda x: x),
 ]