Add SX URLs documentation page, fix layout strapline

New comprehensive documentation for SX URLs at /(applications.(sx-urls))
covering dots-as-spaces, nesting/scoping, relative URLs, keyword ops,
delta values, special forms, hypermedia integration, and GraphSX.
Fix layout tagline: "A" → "The" framework-free reactive hypermedium.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sx/page-functions.sx

@@ -415,6 +415,11 @@
         "retry" '(~example-retry)
         :else '(~example-click-to-load)))))
 
+;; SX URLs (under applications)
+(define sx-urls
+  (fn (slug)
+    '(~sx-urls-content)))
+
 ;; CSSX (under applications)
 (define cssx
   (fn (slug)