docs: Add getting started guide for email authentication setup

docs/06-concepts/11-authentication/01-get-started.md

@@ -0,0 +1,73 @@
+---
+sidebar_label: Get started
+---
+
+# Get started with authentication
+
+When you create a project with `serverpod create`, email sign-in is already built in. The server is configured, the database is ready, and your app is connected to it. You only need to turn on the sign-in screen and run the app.
+
+This guide walks you through that, then shows how to test signing up and signing in.
+
+![Sign-in with Serverpod](/img/authentication/sign-in-widget-device.png)
+
+## Prerequisites
+
+- A project created with `serverpod create` on Serverpod 3.5 or later. For older projects, see [Setup](./setup) first to add the auth module.
+- The Flutter SDK installed, so you can run the app.
+- Docker installed and running, if your project uses a Docker Postgres. Projects on the embedded Postgres option don't need Docker.
+
+## Show the sign-in screen
+
+Your app already includes a sign-in screen. It is just turned off by default. Turn it on with two small edits to your app's `main.dart`.
+
+First, import the screen:
+
+```dart
+import 'screens/sign_in_screen.dart';
+```
+
+Then show it as the home screen. In `MyHomePage.build()`, comment out the existing `body:` line and uncomment the `SignInScreen` block right below it, so it reads:
+
+```dart
+// body: const GreetingsScreen(),
+body: SignInScreen(
+  child: GreetingsScreen(
+    onSignOut: () async {
+      await client.auth.signOutDevice();
+    },
+  ),
+),
+```
+
+That is the only change your app needs.
+
+## Start your project
+
+From the project root, start everything with one command:
+
+```bash
+serverpod start
+```
+
+This sets up the database and starts the server and your app.
+
+The app opens on the sign-in screen. To create an account:
+
+1. Choose to create a new account, enter your email, and continue.
+2. Look in the server console for the verification code. While testing, it is logged instead of emailed:
+
+   ```text
+   [EmailIdp] Registration code (you@example.com): 12345678
+   ```
+
+3. Enter the code, then set a password to finish.
+
+Once you are signed in, the app shows your content with a sign-out button.
+
+## Next steps
+
+- Send real verification emails through an email service. See [Email provider setup](./providers/email/setup).
+- Customize the sign-in screen or build your own. See [Customizing the UI](./providers/email/customizing-the-ui).
+- Change password rules, code length, and rate limits. See [Email configuration](./providers/email/configuration).
+- Require sign-in on your endpoints. See [The basics](./basics#requiring-authentication-on-endpoints).
+- Add more sign-in options, like Google or Apple. See [Identity providers configuration](./setup#identity-providers-configuration).