Clean up, fix, and standardize UISceneDelegate adoption page

[parlough]

• Updates some text to follow our writing guidelines.
• Adjusts and adds semantic breaks in a few locations.
• Fix indentation of tabs, ordered lists, and code blocks to correct nested rendering.
• Updates a few headers for clarity and to be action orientated (with fallback anchors so the old fragments still work).
• Expand a few step introductions for additional clarity.
• Consistently place Swift before Objective-C in tabs.
• Use more separated link definitions and collocate them in the sections they are used in.
• Fix some spacing in code snippets.
• Other misc cleanup and fixes.

Resolves #12572

Staged: https://flutter-docs-prod--docs-pr13491-fix-12572-22r3znch.web.app/release/breaking-changes/uiscenedelegate

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 69296d1):

https://flutter-dev-230821--www-pr13491-fix-12572-pwudbexu.web.app

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 69296d1):

https://flutter-docs-prod--docs-pr13491-fix-12572-22r3znch.web.app

[sfshaza2]

lgtm