docs: add how to preserve module variables

[Timeless0911]

Summary

This pull request introduces several documentation updates and a minor configuration enhancement. The documentation changes improve clarity and add new examples for preserving module variables during builds, while the configuration update adds a new transformer for handling notation diffs in markdown processing.

Documentation Updates:

• Updated the description of source.define in both English (website/docs/en/config/rsbuild/source.mdx) and Chinese (website/docs/zh/config/rsbuild/source.mdx) to highlight its use for injecting environment variables during build time. [1] [2]
• Added a new FAQ section in both English (website/docs/en/guide/faq/features.mdx) and Chinese (website/docs/zh/guide/faq/features.mdx) explaining how to preserve module variables like __webpack_hash__ in build outputs using source.define. [1] [2]
• Fixed minor grammatical issues in the FAQ sections for preserving comments in both English and Chinese documentation. [1] [2]

Configuration Enhancement:

• Added the transformerNotationDiff transformer to the markdown configuration in website/rspress.config.ts to support notation diffs alongside notation highlights. [1] [2]

Related Links

#1000

Checklist

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

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