docs: add Simplified Chinese README translation (添加简体中文说明)

[LelandJin]

Summary

This PR adds a Simplified Chinese translation of the README (README.zh-CN.md) and a language-switch link at the top of README.md, making the project more accessible to Chinese-speaking developers.

本 PR 添加了 README 的简体中文翻译(README.zh-CN.md),并在英文 README.md 顶部加入了语言切换链接,方便中文开发者了解和使用本项目。

• Documentation only — no code changes.
• The translation mirrors the current English README section by section.
• 仅涉及文档,不包含任何代码改动;译文与当前英文 README 逐节对应。

[ccpgames-carbon]

All committers have signed the CLA.

[CCP-Cortege]

Thanks for this, and welcome to the project. Unfortunately, we won't be able to accept this and other similar ones. I'll briefly go over our rationale behind this decision.

Our official language across the engine and company is English, and anything we merge is something we then take on responsibility for keeping accurate as the project changes. We're not set up to maintain translated docs over time, and a translation that's outdated ends up causing more confusion than it solves, so we'd rather not bring it into the repo in the first place. None of that is a comment on the quality of your work, which we appreciate.

Please do keep the translation in your own fork if it's useful to you or to other Chinese-speaking developers. You're very welcome to share it that way, there's just no expectation for it to live in our repo.

If you'd like to keep contributing, code contributions and improvements to the English docs are always welcome. Thanks again for taking the time on this.