refactor: replace docs.langbot.app URLs with link.langbot.app short links

All documentation URLs now go through Cloudflare Bulk Redirects
(link.langbot.app) so future doc path changes won't break
already-released versions.

Short link format: link.langbot.app/{lang}/docs/{topic}
Supported languages: zh, en, ja

src/langbot/pkg/utils/version.py

@@ -203,7 +203,7 @@ class VersionManager:
         try:
             if await self.ap.ver_mgr.is_new_version_available():
                 return (
-                    'New version available:\n有新版本可用，根据文档更新: \nhttps://docs.langbot.app/zh/deploy/update.html',
+                    'New version available:\n有新版本可用，根据文档更新: \nhttps://link.langbot.app/zh/docs/update',
                     logging.INFO,
                 )