docs: add section on testing providers (#9453)

www/apps/resources/references/notification/classes/notification.AbstractNotificationProviderService/page.mdx

@@ -184,3 +184,61 @@ module.exports = defineConfig({
   }
 })
 ```
+
+Make sure to specify the correct channels for your provider in the `channels` option.
+
+---
+
+## 5. Test it Out
+
+### Create Subscriber
+
+To test out the provider, create a subscriber at `src/subscribers/user-created.ts` with the following content:
+
+```ts title="src/subscribers/user-created.ts"
+import { Modules } from "@medusajs/framework/utils"
+import {
+  SubscriberArgs,
+  type SubscriberConfig,
+} from "@medusajs/medusa"
+
+export default async function userCreatedHandler({
+  event: { data },
+  container,
+}: SubscriberArgs<{ id: string }>) {
+  const notificationModuleService = container.resolve(
+    Modules.NOTIFICATION
+  )
+  const userModule = container.resolve(
+    Modules.USER
+  )
+
+  const user = await userModule.retrieveUser(data.id)
+
+  await notificationModuleService.createNotifications({
+    to: user.email,
+    channel: "email",
+    template: "new-user"
+  })
+}
+
+export const config: SubscriberConfig = {
+  event: "user.created",
+}
+```
+
+In the subscriber, you resolve the Notification and User modules. Then, you use the User Module's main service to retrieve the user's details.
+
+Finally, you use the Notification Module's main service to send a notification to the user's email through the `email` channel (assuming that's your provider's channel).
+
+Make sure to replace the value of `template` to the ID of the template in your provider.
+
+### Create User
+
+Use the following command to create a user:
+
+```bash
+npx medusa user -e admin@test.com -p supersecret
+```
+
+After the user is created, the subscriber is executed, sending the notification using your provider.