MOBILE-171: Fix doc paths and clarify Flutter version note in scene example

After UISCENE_MIGRATION.md moved to the repo root, the in-source link in
SceneDelegate.swift and two relative paths inside the doc still pointed at
the old `mindbox_ios/...` and `../example/...` locations. Switched them to
absolute GitHub URLs on `develop`, matching the developers.mindbox.ru
callout and the in-source comment.

Also rewrote the SceneDelegate.swift header comment: `FlutterSceneDelegate`
itself needs Flutter ≥ 3.35, but the example app pins to ≥ 3.41 because of
`FlutterImplicitEngineDelegate` in AppDelegate.

Author: justSmK

UISCENE_MIGRATION.md

@@ -43,7 +43,7 @@ forward them from your scene delegate.
 ## What to add in your scene delegate
 
 Copy
-[`example/flutter_example/ios/Runner/SceneDelegate.swift`](../example/flutter_example/ios/Runner/SceneDelegate.swift)
+[`example/flutter_example/ios/Runner/SceneDelegate.swift`](https://github.com/mindbox-cloud/flutter-sdk/blob/develop/example/flutter_example/ios/Runner/SceneDelegate.swift)
 into your `Runner` target. The two relevant calls:
 
 ```swift
@@ -103,7 +103,7 @@ nothing to do with the host app's scene flow. They need no changes.
 
 ## Reference implementation
 
-[`example/flutter_example/ios/Runner`](../example/flutter_example/ios/Runner)
+[`example/flutter_example/ios/Runner`](https://github.com/mindbox-cloud/flutter-sdk/tree/develop/example/flutter_example/ios/Runner)
 is fully migrated and serves as a working reference for both the
 Flutter-side and Mindbox-side parts of the migration.
 

example/flutter_example/ios/Runner/SceneDelegate.swift

@@ -2,9 +2,12 @@ import UIKit
 import Flutter
 import Mindbox
 
-// Sample SceneDelegate forwarding scene events to Mindbox. Requires Flutter >= 3.35.
+// Sample SceneDelegate forwarding scene events to Mindbox.
+// `FlutterSceneDelegate` itself requires Flutter >= 3.35; this example app
+// also relies on `FlutterImplicitEngineDelegate` in AppDelegate, which is
+// Flutter >= 3.41.
 // Mindbox-side notes:
-// https://github.com/mindbox-cloud/flutter-sdk/blob/develop/mindbox_ios/UISCENE_MIGRATION.md
+// https://github.com/mindbox-cloud/flutter-sdk/blob/develop/UISCENE_MIGRATION.md
 @available(iOS 13.0, *)
 class SceneDelegate: FlutterSceneDelegate {