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/sx/docs-content.sx

@@ -76,6 +76,15 @@
         "sx provides ~80 built-in pure functions. They work identically on server (Python) and client (JavaScript).")
       (div :class "space-y-6" prims))))
 
+(defcomp ~docs-special-forms-content (&key forms)
+  (~doc-page :title "Special Forms"
+    (~doc-section :title "Syntactic constructs" :id "special-forms"
+      (p :class "text-stone-600"
+        "Special forms are syntactic constructs whose arguments are NOT evaluated before dispatch. Each form has its own evaluation rules — unlike primitives, which receive pre-evaluated values. Together with primitives, special forms define the complete language surface.")
+      (p :class "text-stone-600"
+        "Forms marked with a tail position enable " (a :href "/essays/tco" :class "text-violet-600 hover:underline" "tail-call optimization") " — recursive calls in tail position use constant stack space.")
+      (div :class "space-y-10" forms))))
+
 (defcomp ~docs-css-content ()
   (~doc-page :title "On-Demand CSS"
     (~doc-section :title "How it works" :id "how"