add redirect from /sitemap to /sitemap.xml in web.lisp (defroute).

This is just a 'catch-all' defroute.

src/web.lisp

@@ -1447,6 +1447,9 @@
 			            :nav-menu ,(nera:nav-menu-slugs)
 			            :system-data ,(nera:system-data)))))
 
+(defroute ("/sitemap" :method :GET) ()
+  (redirect "/sitemap.xml"))
+
 (defroute ("/sitemap.xml" :method :GET) ()
   (setf (getf (response-headers *response*) :content-type) "application/xml")
   (with-output-to-string (stream)