docs(guides): add getting-started style guide for experiments.css

[phoekerson]

Feature to document

experiments.css Native CSS support in webpack 5/6
Related configuration reference: https://webpack.js.org/configuration/experiments/#experimentscss

Author(s)

@alexander-akait (core maintainer driving the experiments.css implementation)

Additional information

The webpack 2026 roadmap blog post highlights native CSS support via
experiments.css as one of the most significant upcoming features for
webpack 6. The configuration reference page documents the available
options, but no guide under src/content/guides/ walks developers
through the full workflow end-to-end.

A dedicated guide is needed to cover:

• Enabling experiments.css: true in webpack.config.js
• Importing CSS files directly from JavaScript source files
• CSS Modules support with experiments.css
• Production build setup and comparison with mini-css-extract-plugin
• Current experimental status and known limitations
• Migration path for users currently using css-loader +
  mini-css-extract-plugin

This guide would complement the existing configuration reference and
make the feature accessible to the large number of developers who learn
webpack through its guides rather than its configuration docs.

Proposed file: src/content/guides/native-css.mdx

[ ] I am willing to work on this issue and submit a pull request.

[alexander-akait]

Yes, feel free to improve it, it will help