feat: add tip for auto sign-in in next app (#1311)

Author: wangsijie

docs/quick-starts/framework/next-app-router/README.mdx

@@ -106,6 +106,37 @@ HTTP does not allow setting cookies after streaming starts, `getOrganizationToke
 
 <ExternalStorage />
 
+## Automatic sign-in redirect on unauthorized \{#automatic-sign-in-redirect\}
+
+:::tip
+The `signIn` helper modifies cookies to establish the sign-in session, so it cannot be invoked directly in a React Server Component (RSC). To trigger sign-in automatically when an RSC detects an unauthorized user, call `signIn` in a dedicated route handler and redirect to that route.
+:::
+
+```ts title="app/sign-in/route.ts"
+import { signIn } from '@logto/next/server-actions';
+import { logtoConfig } from '../../logto';
+
+export async function GET() {
+  await signIn(logtoConfig);
+}
+```
+
+```tsx title="app/protected/page.tsx"
+import { getLogtoContext } from '@logto/next/server-actions';
+import { redirect } from 'next/navigation';
+import { logtoConfig } from '../logto';
+
+export default async function ProtectedPage() {
+  const { isAuthenticated } = await getLogtoContext(logtoConfig);
+
+  if (!isAuthenticated) {
+    redirect('/sign-in');
+  }
+
+  return <div>Protected content</div>;
+}
+```
+
 ## Further readings \{#further-readings}
 
 <FurtherReadings />