docs: add guide to reflect CSS rule structure changes

[chenjiahan]

Summary

Added a new section explaining that the built-in CSS rule is now split into three oneOf branches: CSS_MAIN, CSS_RAW, and CSS_INLINE, with instructions to move custom loaders to the CSS_MAIN branch.

Related Links

• feat!: convert the built‑in CSS rule to oneOf structure #7037

Checklist

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

[gemini-code-assist]

Summary of Changes

Hello @chenjiahan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the upgrade guide from Rsbuild v1 to v2 by documenting significant changes to how CSS rules are structured. It introduces a detailed explanation of the new oneOf branching for CSS processing, including CSS_MAIN, CSS_RAW, and CSS_INLINE, and provides clear instructions for users to adapt their custom loader configurations to these new branches. This update is crucial for ensuring a smooth migration experience for developers.

Highlights

• CSS Rule Structure Update: The built-in CSS rule is now split into three oneOf branches: CSS_MAIN for normal transforms, CSS_RAW for ?raw imports, and CSS_INLINE for ?inline imports.
• Custom Loader Migration Guidance: Users who previously added loaders on CHAIN_ID.RULE.CSS should now move them to the CSS_MAIN branch, with an example provided in the documentation.
• Plugin Consistency: Similar oneOf changes apply to the built-in rules in Less, Sass, and Stylus plugins, requiring corresponding updates for customizations.
• JS Rule Documentation Refinement: A minor clarification was made in the JS rule documentation, specifying that custom JS loaders should be moved to the JS_MAIN branch.

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[Copilot]

Pull request overview

This PR updates the v1-to-v2 upgrade guides to document the new oneOf-based CSS rule structure and clarify how to migrate custom loaders for both JS and CSS.

Changes:

• Clarified JS upgrade instructions to explicitly direct custom loaders from CHAIN_ID.RULE.JS to the JS_MAIN branch.
• Added a new section documenting the split of the built-in CSS rule into CSS_MAIN, CSS_RAW, and CSS_INLINE oneOf branches.
• Provided concrete configuration diffs showing how to move custom CSS loaders and referenced equivalent changes for Less/Sass/Stylus plugin rules.

Reviewed changes

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

File	Description
website/docs/zh/guide/upgrade/v1-to-v2.mdx	Updates the JS section wording and adds a new CSS section (in Chinese) explaining the oneOf branches and how to migrate custom loaders.
website/docs/en/guide/upgrade/v1-to-v2.mdx	Mirrors the JS wording clarification and adds the English CSS rule oneOf documentation and migration example.

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[gemini-code-assist]

Code Review

This pull request updates the v1-to-v2 upgrade guide to document changes in the built-in JS and CSS rule structures. The update for the JS rule is clear and accurate. However, the new documentation for the CSS rule structure is inconsistent with the provided source code. The documentation describes a oneOf structure for CSS rules, but the implementation uses separate top-level rules. I've left critical comments on both the English and Chinese documentation files to highlight this discrepancy and suggest corrections to ensure the documentation is accurate.