Add additional instructions to UIScene Migration

[vashworth]

Description of what this PR is changing or adding, and why:

Adds instructions on how to add a SceneDelegate and how to use auto-migrator

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

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[vashworth]

cc @LongCatIsLooong

[gemini-code-assist]

Summary of Changes

Hello @vashworth, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the documentation for migrating Flutter applications to UIScene. It provides explicit instructions for leveraging the experimental auto-migration tool and offers a step-by-step guide for developers who need to manually create and configure a custom SceneDelegate in their iOS projects. The goal is to make the migration process more accessible and understandable for a wider range of use cases.

Highlights

• Auto-Migration Instructions: Detailed steps have been added for using the experimental auto-migration feature for UIScene, including how to enable it via flutter config and what to expect from the build process.
• Optional SceneDelegate Creation: New instructions guide users on how to manually create a custom SceneDelegate (subclassing FlutterSceneDelegate) for both Swift and Objective-C projects, along with updating the Info.plist.
• Documentation Enhancement: The existing uiscenedelegate.md documentation has been significantly expanded to provide clearer and more comprehensive guidance for Flutter app developers migrating to UIScene.

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[gemini-code-assist]

Code Review

This pull request enhances the UISceneDelegate migration guide by adding clear, step-by-step instructions for both the auto-migration process and the manual creation of a SceneDelegate. The new sections are well-written and will be very helpful for developers. I've only found a minor typo in the new content, which I've suggested a fix for.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 7b62552):

https://flutter-docs-prod--pr12592-uiscene-migrate-apps-0cbj7oyu.web.app

[sfshaza2]

lgtm, with a few minor mods