Fix geography index: restore default content and add page gutters

The geography page function returned nil instead of the index-content
component, and the index layout was missing the standard doc page wrapper.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/page-functions.sx

@@ -61,7 +61,7 @@
 
 (define geography
   (fn (content)
-    (if (nil? content) nil content)))
+    (if (nil? content) '(~geography/index-content) content)))
 
 (define applications
   (fn (content)