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/sxc/pages/sx_router.py

@@ -37,6 +37,7 @@ _PAGE_FNS = {
     "doc", "spec", "explore", "bootstrapper", "test",
     "reference", "reference-detail", "example",
     "cssx", "protocol", "essay", "philosophy", "plan",
+    "sx-urls",
 }
 
 # All known function names (don't auto-quote these)