docs: add avatar drivers guide for User extender

docs/extend/avatars.md

@@ -0,0 +1,74 @@
+# Avatar Drivers
+
+Flarum allows new avatar drivers to be added through extenders. By default, Flarum includes two drivers: the Default driver (which uses uploaded avatars) and the Gravatar driver (which generates avatars from user email addresses).
+
+To create your own avatar driver, you will need to create a class implementing `Flarum\User\Avatar\DriverInterface`. This allows extensions to integrate custom avatar providers from external services like Discord, GitHub, Steam, or any custom service.
+
+## How It Works
+
+When Flarum needs to display a user's avatar, it resolves the avatar URL through a fallback chain:
+
+1. **Custom uploaded avatar** - If the user has uploaded an avatar, use that file
+2. **Custom avatar URL** - If a full URL (with `://`) is set in the database
+3. **Configured avatar driver** - Call the active driver's `avatarUrl()` method
+4. **Null** - Falls back to default UI avatar (initials)
+
+Your driver is only called when the user has no uploaded avatar or custom URL. This means users can always override your driver by uploading their own avatar. The `$user->avatar_url` accessor handles this resolution automatically. You can check if a user has a custom avatar using `$user->original_avatar_url`.
+
+## Creating a Driver
+
+The `DriverInterface` has a single method which receives a `User` model and returns either a URL string or null:
+
+```php
+namespace Acme\Avatars\Driver;
+
+use Flarum\User\Avatar\DriverInterface;
+use Flarum\User\User;
+
+class DiscordAvatarDriver implements DriverInterface
+{
+    public function avatarUrl(User $user): ?string
+    {
+        $discordId = $user->discord_id;
+        $avatarHash = $user->discord_avatar_hash;
+
+        if (! $discordId || ! $avatarHash) {
+            return null;
+        }
+
+        return "https://cdn.discordapp.com/avatars/{$discordId}/{$avatarHash}.png?size=200";
+    }
+}
+```
+
+The `avatarUrl()` method receives the full User model, so you can access any user properties including custom attributes you have added via [migrations](database.md).
+
+Drivers should return `null` when they cannot provide an avatar URL. This allows Flarum to fall back to the default avatar display.
+
+## Registering a Driver
+
+To register avatar drivers, use the `Flarum\Extend\User` extender in your extension's `extend.php` file:
+
+```php
+use Flarum\Extend;
+use Acme\Avatars\Driver\DiscordAvatarDriver;
+
+return [
+    // Other extenders
+    (new Extend\User())
+        ->avatarDriver('discord', DiscordAvatarDriver::class),
+    // Other extenders
+];
+```
+
+The first parameter is a unique identifier for your driver. This identifier is stored in the `avatar_driver` setting when an administrator selects it from the admin panel, so it should be unique across all extensions.
+
+## Admin Configuration
+
+Once registered, avatar drivers will automatically appear in the admin dashboard under **Basics → Avatar Driver**. The dropdown will only appear when two or more drivers are registered. Administrators can select which driver should be active for users who have not uploaded custom avatars.
+
+Flarum automatically adds your driver to the admin settings page when you register it. No additional frontend code is required.
+
+The selected driver identifier is stored in the `avatar_driver` setting. If the setting is empty or refers to an unregistered driver, Flarum will use the Default driver as a fallback.
+
+For a real-world example, see Flarum's built-in [Gravatar driver](https://github.com/flarum/framework/blob/main/framework/core/src/User/Avatar/GravatarDriver.php).

sidebars.js

@@ -128,6 +128,7 @@ module.exports = {
       items: [
         'extend/api-throttling',
         'extend/assets',
+        'extend/avatars',
         'extend/console',
         'extend/database',
         'extend/extending-extensions',