Add Special Forms docs page at /docs/special-forms

Parses special-forms.sx spec into categorized form cards with syntax,
description, tail-position info, and highlighted examples. Follows the
same pattern as the Primitives page: Python helper returns structured
data, .sx components render it.

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

shared/sx/ref/boundary.sx

@@ -293,6 +293,11 @@
   :returns "dict"
   :service "sx")
 
+(define-page-helper "special-forms-data"
+  :params ()
+  :returns "dict"
+  :service "sx")
+
 (define-page-helper "reference-data"
   :params (slug)
   :returns "dict"