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/geography/index.sx

@@ -2,7 +2,7 @@
 ;; Describes the rendering pipeline: OCaml evaluator → wire formats → client
 
 (defcomp ~geography/index-content () :affinity :server
-  (<>
+  (div :class "max-w-4xl mx-auto px-6 pb-8 pt-4"
     (h2 :class "text-3xl font-bold text-stone-800 mb-4" "Geography")
     (p :class "text-lg text-stone-600 mb-8"
       "Where code runs and how it gets there. Geography maps the rendering pipeline from server-side evaluation through wire formats to client-side hydration.")

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)