docs: prefer to use the postcss plugin of unocss

[chenjiahan]

Summary

The PR reverts #4287 and adds the @unocss/postcss guide back, as @unocss/webpack has some limitations.

Related Links

• [Bug]: unocss rspack plugin hmr not work #4615
• [Bug]: UnoCSS Hot Reload doesn't work fine with Vue Project rspack#10978

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR reverts the previous Webpack-based UnoCSS integration and restores the PostCSS plugin approach in the documentation.

• Switched from @unocss/webpack to unocss & @unocss/postcss in install instructions
• Replaced Rsbuild/Webpack plugin setup with a postcss.config.mjs example
• Updated config and CSS import steps in both Chinese and English guides

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
website/docs/zh/guide/styling/unocss.mdx	用 PostCSS 插件取代 @unocss/webpack 集成示例，并更新安装与配置步骤
website/docs/en/guide/styling/unocss.mdx	Updated to use PostCSS plugin instead of Webpack, adjusted commands and setup

Comments suppressed due to low confidence (3)

website/docs/zh/guide/styling/unocss.mdx:5

• 建议为 PostCSS 插件 添加指向 @unocss/postcss npm 包的链接，以便读者快速访问该插件的详细信息。

你可以通过 PostCSS 插件来在 Rsbuild 中接入 UnoCSS。

website/docs/zh/guide/styling/unocss.mdx:27

• 考虑在此处添加一条注释，提示读者 Rsbuild 已集成 Autoprefixer，可以直接只注册 UnoCSS 插件。

## 配置 UnoCSS

website/docs/en/guide/styling/unocss.mdx:9

• Consider adding hyperlinks to the npm pages for unocss and @unocss/postcss so readers can easily access the package documentation.

You need to install `unocss` and `@unocss/postcss` first.