docs: drop the v5-design design note from the site

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DgcYQy7Mnh2afkmSg295uU

Author: Toilal

docs/v5-design.md

@@ -12,7 +12,6 @@ nav = [
     { "Usage" = "usage.md" },
     { "Rules & processors" = "rules.md" },
     { "API reference" = "api.md" },
-    { "v5 design notes" = "v5-design.md" },
 ]
 
 [project.theme]