chore: create docs-en-improvement.prompt.yml

[chenjiahan]

Summary

This pull request adds a new prompt configuration file to guide improvements to the English documentation.

Related

• https://docs.github.com/en/github-models/use-github-models/storing-prompts-in-github-repositories

Checklist

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

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[Copilot]

Pull Request Overview

Adds a new prompt configuration to standardize how contributors improve the English documentation, including tone, scope, and PR guidelines.

• Introduces a system message with concrete editing rules and style guidance.
• Specifies a default model for running the prompt.

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