[doc] Migrate old Rails 3 example to a Rails 7.2 one

Generally works via Warbler and Tomcat 9, although not sure if the Rack `forward_to` for JSPs is actually functional. The SnoopController is working though, so seems better than the previous example.

Author: chadlwilson

examples/Kirkfile

@@ -1,4 +1,4 @@
-rack "rails3/config.ru" do
+rack "rails7/config.ru" do
   listen 9090
 end