Add M3E migration guideline

[QuncCccccc]

This PR is to add a m3e migration guide to docs/ecosystem/material_ui.

Related to Bring Material 3 Expressive to Flutter
Followup: create a skill that point to this doc to help agent during migration: flutter/packages#12818

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in [flutter/website] (or verified none is needed).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request adds a comprehensive Material 3 Expressive component migration checklist to the ecosystem documentation and updates the main README index to link to it. The review feedback suggests running Dart scripts and tests from the package directory rather than the repository root to ensure correct dependency resolution, and correcting an example path to match the structure of the flutter/packages repository.

[Piinks]

Thanks for this @QuncCccccc!

Here are a few suggestions and minor discrepancies to consider before landing:

• Section 2 - Template Skeleton
  • In packages/material_ui/tool/gen_defaults/templates/template.dart, TokenTemplate.generateFile asserts that the output of generateContents contains class $className. Returning only a comment triggers an AssertionError. Including class $className { ... } in the skeleton avoids this.
• Section 12 - Generator Tests
  • Running dart run test test in tool/gen_defaults on a clean checkout requires flutter pub get first because gen_defaults has a dependency on the Flutter SDK (flutter: sdk: flutter). Using flutter test directly avoids this dependency resolution issue.
• Section 10 - Example Doc Pattern:
  • In packages/material_ui, doc samples currently use <callout-box>, {@macro material_ui.dartpad_guide}, and {@example /example/lib/...#body}, rather than {@tool dartpad} and See code in .... Updating the snippet to reflect that pattern will keep new docs consistent.
• Section 11 - Pending Changelog Terms
  • For manual YAML creation, the version bump values are major, minor, patch, or skip. The values bugfix and next are CLI options for flutter_plugin_tools update-release-info. Clarifying this distinction avoids invalid entries like version: bugfix in manual YAML files.
• Section 6 - Reference Link
  • Mentioning that the IconButton migration in flutter/packages#12832 is the concrete reference implementation for ButtonStyle.sizeVariant and the button variants provides great context for contributors while that PR is in flight.
• Section 4 - Theme Data
  • Add copyWith to the list of methods to update when adding variant to component theme data classes.