chore: add `中文 - zh-CN` language support

### Current Limitation

Currently, the Asgardeo JavaScript SDK does not have support for **Chinese (Simplified - 中文 - zh-CN)**. Users who prefer Simplified Chinese will not see UI components translated, limiting accessibility for Chinese-speaking users.

> [!Important]
> Contributors for this task should be **native Chinese speakers or have professional fluency** to ensure accurate, culturally appropriate translations.

### Suggested Improvement

Add **Chinese (`zh-CN`) language support** to the `@asgardeo/i18n` package.

**Implementation Plan:**

1. Create a new locale file:

Follow https://github.com/asgardeo/javascript/blob/main/CONTRIBUTING.md#contributing-to-the-internalization-i18n-package

2. Add documentation for usage:
- Update the SDK docs: https://wso2.com/asgardeo/docs/sdks/react/overview/

### Please select the package issue is related to

@asgardeo/react, @asgardeo/nextjs

### Version

current

### Reporter Checklist

- [x] I have searched the existing issues and this is not a duplicate.
- [x] I have provided all the necessary information.
- [x] I have verified the improvement is not available in the latest version of the package.

CONTRIBUTING.md の internalization/i18n パッケージへの貢献に関するセクションから始め、@asgardeo/i18n の locale 構造を確認します。簡体字中国語 (zh-CN) のサポートを追加し、リンク先の概要ページにある Asgardeo React SDK のドキュメントを更新します。完了には、正確な翻訳と使用方法のドキュメント化が含まれている必要があります。

索引モデルが issue の本文から書いたものです。