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>

sx/sxc/pages/docs.sx

@@ -44,6 +44,8 @@
     "evaluator" (~docs-evaluator-content)
     "primitives" (~docs-primitives-content
                    :prims (~doc-primitives-tables :primitives (primitives-data)))
+    "special-forms" (~docs-special-forms-content
+                      :forms (~doc-special-forms-tables :forms (special-forms-data)))
     "css" (~docs-css-content)
     "server-rendering" (~docs-server-rendering-content)
     :else (~docs-introduction-content)))