Update built-in Kotlin migration doc

[jesswrd]

Updated the name of doc from "Upgrade to AGP 9 Migration Docs" to "Built-in Kotlin Migraction Docs."

I added the following:
-migration instructions for plugin authors
-migration isntructions for app develoeprs working on add-to-app host apps

I also refactored the docs to have the same structure as the swift package manager migration docs here. They seem more readable.

These website docs links here are referenced when developers receive error logs here.

Related to: flutter/flutter#181557

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[gemini-code-assist]

Code Review

This pull request updates the breaking changes documentation by replacing the Android Gradle Plugin 9.0.0 migration guide with a more comprehensive guide focused on migrating Flutter Android projects to built-in Kotlin. The new documentation provides detailed instructions for apps, plugins, and host apps using both Kotlin and Groovy DSLs. Review feedback focuses on improving the documentation's adherence to technical writing standards, including the correct capitalization of "Flutter" and "Android," fixing grammatical errors in pluralization, and removing first-person pronouns and informal phrasing to align with style guidelines.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 58d6ed8):

https://flutter-docs-prod--pr13254-update-built-in-kotlin-migr-k8k1lhh5.web.app

[gemini-code-assist]

Code Review

This pull request replaces the previous Android Gradle Plugin 9.0.0 migration guide with a comprehensive set of documents for migrating Flutter projects to built-in Kotlin. The new documentation includes dedicated guides for app developers and plugin authors, along with updated site navigation. Feedback identifies a missing link definition for Issue #184839 and suggests several improvements to the clarity of DSL block descriptions and the directness of internal links.

[sfshaza2]

Overall, lgtm! Just a few formatting issues.