docs: clarify methods to access environment context

[chenjiahan]

Summary

Added two ways to access the environment context: via Rsbuild's plugin hooks and the Environment API, with corresponding links for better guidance.

Related Links

Checklist

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

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[Copilot]

Pull Request Overview

This PR clarifies how to access the environment context by updating the documentation in both the Chinese and English versions.

• Updated the explanation to include two methods of accessing the environment context via Rsbuild's Plugin hooks and the Environment API.
• Modified type annotations to use a link reference for EnvironmentContext.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
website/docs/zh/api/javascript-api/environment-api.mdx	Updated documentation text and list format for clearer guidance on accessing environment context.
website/docs/en/api/javascript-api/environment-api.mdx	Updated documentation text and list format for a clearer explanation of accessing environment context.