[zh] Add Simplified Chinese translation for README

### Motivation

🤗 Datasets is a fundamental library widely used by researchers, data scientists, and developers worldwide, with a large and active Chinese-speaking developer and research community.

Providing an accurate, idiomatic Simplified Chinese translation (`README.zh.md`) along with a language switcher will greatly lower the onboarding barrier for developers and students in Chinese-speaking regions, helping them easily adopt 🤗 Datasets for multi-modal data loading, streaming, and model training.

### Proposed Changes

- Translate the root `README.md` into Simplified Chinese (`README.zh.md`) covering all sections (Key Features, Installation, Quick Start, Streaming, Multi-modal data, Local files, Python objects, Core classes, Hub uploads, and Disclaimers).
- Add bidirectional language switcher (`English` · `简体中文`) in both `README.md` and `README.zh.md`.
- Include community maintenance commitment to ensure long-term synchronization with upstream documentation updates.

I have already prepared the translation PR and will link it to this issue.

Start with the root README.md and review each listed section, including installation, quick start, streaming, and Hub uploads. Check the existing README structure and links before reviewing the Simplified Chinese translation. Done means README.zh.md covers all sections and both files provide working English and 简体中文 language switches.

Written by the indexing model from the issue text.