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

web/src/app/home/components/home-sidebar/HomeSidebar.tsx

@@ -1423,12 +1423,12 @@ export default function HomeSidebar({
                           localStorage.getItem('langbot_language');
                         if (language === 'zh-Hans' || language === 'zh-Hant') {
                           window.open(
-                            'https://docs.langbot.app/zh/insight/guide',
+                            'https://link.langbot.app/zh/docs/guide',
                             '_blank',
                           );
                         } else {
                           window.open(
-                            'https://docs.langbot.app/en/insight/guide',
+                            'https://link.langbot.app/en/docs/guide',
                             '_blank',
                           );
                         }