Add Continuations essay to SX docs

Covers server-side (suspendable rendering, streaming, error boundaries),
client-side (linear async flows, wizard forms, cooperative scheduling,
undo), and implementation path from the existing TCO trampoline. Updates
TCO essay's continuations section to link to the new essay instead of
dismissing the idea. Fixes "What sx is not" to acknowledge macros + TCO.

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

sx/content/pages.py

@@ -73,6 +73,8 @@ ESSAYS_NAV = [
     ("Client Reactivity", "/essays/client-reactivity"),
     ("SX Native", "/essays/sx-native"),
     ("The SX Manifesto", "/essays/sx-manifesto"),
+    ("Tail-Call Optimization", "/essays/tail-call-optimization"),
+    ("Continuations", "/essays/continuations"),
 ]
 
 MAIN_NAV = [