docs: add section about extending plugin API

[chenjiahan]

Summary

Add documentation for exposing custom methods to Rsbuild plugins using rsbuild.expose() and accessing them via api.useExposed().

Checklist

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

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[Copilot]

Pull Request Overview

This PR adds documentation for extending the Rsbuild plugin API by exposing custom methods from the Rsbuild instance to plugins. The documentation explains how to use rsbuild.expose() to share custom functionality and api.useExposed() to access it from within plugins.

• Adds new section "Extending the Plugin API" to plugin development documentation
• Provides code examples demonstrating the expose/useExposed pattern for sharing state and methods
• Includes both English and Chinese translations

Reviewed Changes

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

File	Description
website/docs/zh/plugins/dev/index.mdx	Added Chinese documentation for extending plugin API with rsbuild.expose() and api.useExposed()
website/docs/en/plugins/dev/index.mdx	Added English documentation for extending plugin API with rsbuild.expose() and api.useExposed()

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