docs: deprecate mini-css-extract-plugin in favor of webpack's built-in CSS support

[alexander-akait]

Summary

webpack extracts CSS into separate files itself: experiments.css defaults to "auto" since 5.109.0, extraction is the built-in default, and output.cssFilename / output.cssChunkFilename replace this plugin's filename and chunkFilename. Adds a deprecation notice at the top of the README linking the Native CSS guide, its migration guide and the codemod.

What kind of change does this PR introduce?

docs

Did you add tests for your changes?

n/a — README and .cspell.json only, no code change.

Does this PR introduce a breaking change?

No.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

Nothing further — https://webpack.js.org/plugins/mini-css-extract-plugin/ is generated from this README, so the notice appears there on the next docs build. The /plugins index already marks the plugin deprecated.

Use of AI

Yes — Claude Code drafted the notice and checked every linked anchor against the guides in webpack.js.org and the experiments.css version against webpack's schema and changelog. Reviewed and edited by me before pushing.

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.31%. Comparing base (d50f981) to head (f07823c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1166   +/-   ##
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  Coverage   85.31%   85.31%           
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  Files           5        5           
  Lines        1049     1049           
  Branches      314      314           
=======================================
  Hits          895      895           
  Misses        130      130           
  Partials       24       24           

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