docs: add section on global styles in CSS Modules

[chenjiahan]

Summary

Updates the documentation for CSS Modules to include information about using global styles with the :global() pseudo-class selector. The changes provide examples and explanations for combining local and global selectors, as well as nesting :global().

Related Links

• https://github.com/css-modules/css-modules/blob/c7bf6c02472fdce4dd8e569abc10d2aeb234e1c2/docs/composition.md#exceptions
• [Bug]: When PostCSS processes SCSS files, it merges selectors into :is(...), but only the first class is being processed.. #5405

Checklist

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

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[Copilot]

Pull Request Overview

This PR updates the CSS Modules documentation to include guidance on using global styles with the :global() pseudo-class selector. Key changes include:

• Added a new section on global styles in both Chinese and English documentation.
• Provided examples demonstrating local versus global selectors.
• Included a nested :global() usage example for scoped overrides.

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/css-modules.mdx	Added a "全局样式" section with examples on using :global() selectors.
website/docs/en/guide/styling/css-modules.mdx	Added a "Global styles" section with corresponding examples.