docs: create a Swift version example of the ios-notification-images doc (#8185)

* docs: create a Swift version example of the ios-notification-images doc
* fix: fix syntax highlighting in ios-notification-images
* style(lint): result of `yarn lint:markdown --write`

---------

Co-authored-by: Mike Hardy <[email protected]>

Author: petetnt

docs/messaging/ios-notification-images.md

@@ -43,7 +43,9 @@ end
 
 ![step-2](/assets/docs/messaging/ios-notification-images-step-2.gif)
 
-### Step 3 - Use the extension helper
+### Step 3 - Use the extension helper (Objective-C)
+
+> If you selected to create your extension as a Swift project, jump to the next section.
 
 At this point everything should still be running normally. This is the final step which is invoking the extension helper.
 
@@ -68,6 +70,34 @@ At this point everything should still be running normally. This is the final ste
 
 ![step-3](/assets/docs/messaging/ios-notification-images-step-3.gif)
 
+### Step 3 - Use the extension helper (Swift)
+
+At this point everything should still be running normally. This is the final step which is invoking the extension helper.
+
+- From the navigator select your `ImageNotification` extension
+- Open the `NotificationService.swift` file
+- At the top of the file import `Firebase` right after the `NotificationService` as shown below
+
+```diff
+import UserNotifications
++ import Firebase
+
+class NotificationService: UNNotificationServiceExtension {
+```
+
+- then replace everything from line 19 to 23 with the extension helper
+
+```diff
+        if let bestAttemptContent = bestAttemptContent {
+-            // Modify the notification content here...
+-            bestAttemptContent.title = "\(bestAttemptContent.title) [modified]"
+-
+-            contentHandler(bestAttemptContent)
++            Messaging.serviceExtension()
++               .populateNotificationContent(bestAttemptContent, withContentHandler: contentHandler)
+        }
+```
+
 ## All done
 
 Run the app and check it builds successfully – **make sure you have the correct target selected**. Now you can use the [Notifications composer](https://console.firebase.google.com/u/0/project/_/notification) to test sending notifications with an image (`300KB` max size). You can also create custom notifications via [`FCM HTTP`](https://firebase.google.com/docs/cloud-messaging/http-server-ref) or [`firebase-admin`](https://www.npmjs.com/package/firebase-admin). Read this page to send [messages from a server](/messaging/server-integration).