[zh] Translating docs to Simplified Chinese

Hi!

Let's bring the documentation to all the Chinese-speaking community 🌐.

This issue tracks the Simplified Chinese documentation effort for 🤗 Diffusers, starting with the core `README.zh.md` and language navigation.

- [x] `README.zh.md` translation and reciprocal switcher (PR #14707)

cc @stevhliu @sayakpaul

Start with README.zh.md and the language navigation described in the issue; review PR #14707 to understand the completed translation and reciprocal switcher. Identify the remaining Simplified Chinese documentation work in the tracker, and consider it done when the relevant translations and navigation updates are complete.

Scritto dal modello di indicizzazione a partire dal testo della issue.