Add "The SX Manifesto" essay to sx docs app

A satirical essay in the style of The Communist Manifesto, recasting
the historic struggle between bourgeoisie and proletariat as the war
between HTML, JS, and CSS — with frameworks as petty-bourgeois lackeys
and s-expressions as the revolutionary force that abolishes the
language distinction itself.

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

sx/content/pages.py

@@ -72,6 +72,7 @@ ESSAYS_NAV = [
     ("On-Demand CSS", "/essays/on-demand-css"),
     ("Client Reactivity", "/essays/client-reactivity"),
     ("SX Native", "/essays/sx-native"),
+    ("The SX Manifesto", "/essays/sx-manifesto"),
 ]
 
 MAIN_NAV = [