docs: add platform-specific assets section to assets guide

[lamek]

Adds a new "Platform-specific assets" section to the assets documentation,
explaining how to configure assets for specific platforms in pubspec.yaml.
This feature is available starting from Flutter 3.41.0.
Also adds reference links pointing to the detailed assets field in the
Flutter pubspec options page.

#13260

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  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request adds documentation for the platform-specific assets feature introduced in Flutter 3.41.0, including a YAML configuration example and reference links. The reviewer suggested a minor improvement to shorten a line to comply with the 80-character limit and make the phrasing more direct.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit e5f6d95):

https://flutter-docs-prod--docs-pr13666-fix-13260-yhrafo9x.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit e5f6d95):

https://flutter-dev-230821--www-pr13666-fix-13260-1zdjy51o.web.app