docs: improve documentation for file size diff

[chenjiahan]

Summary

Update the documentation for the printFileSize.diff option to provide clearer explanations and examples:

Checklist

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

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[Copilot]

Pull request overview

This pull request improves the documentation for the printFileSize.diff configuration option by providing more detailed explanations and examples across English and Chinese documentation files, as well as updating the TypeScript JSDoc comments.

Key Changes:

• Enhanced explanation of how the diff feature works (snapshot recording and comparison)
• Added comprehensive examples showing the output format with size deltas
• Improved clarity of when the diff information becomes available (starting from second build)
• Added version history section documenting the feature's introduction in v1.6.13

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
website/docs/en/config/performance/print-file-size.mdx	Updated English documentation with clearer descriptions and examples for the diff option, including output format and snapshot storage details
website/docs/zh/config/performance/print-file-size.mdx	Added complete Chinese documentation for the diff option with translated content matching the English version structure
packages/core/src/types/config.ts	Enhanced JSDoc comment for the diff property with more detailed explanation of the snapshot mechanism

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