docs: add better auth sidebar link

Author: onmax

docs/app/app.config.ts

@@ -11,6 +11,11 @@ export default defineAppConfig({
       { label: 'Convex', to: 'https://docs.convex.dev', target: '_blank' },
     ],
   },
+  navigation: {
+    links: [
+      { title: 'Better Auth', to: 'https://nuxt-better-auth.onmax.me/integrations/convex', target: '_blank', icon: 'i-heroicons-arrow-top-right-on-square' },
+    ],
+  },
   ui: {
     colors: {
       primary: 'amber',