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/nav-data.sx

@@ -379,6 +379,7 @@
         {:label "Testing" :href "/(language.(test))" :children testing-nav-items})}
      {:label "Applications" :href "/(applications)"
       :children (list
+        {:label "SX URLs" :href "/(applications.(sx-urls))"}
         {:label "CSSX" :href "/(applications.(cssx))" :children cssx-nav-items}
         {:label "Protocols" :href "/(applications.(protocol))" :children protocols-nav-items})}
      {:label "Etc" :href "/(etc)"