docs: add Simplified Chinese translation (README.zh.md)

[JasonYeYuhe]

What does this PR do?

Fixes #14711

This PR introduces a comprehensive Simplified Chinese translation (README.zh.md) for 🤗 Diffusers to improve accessibility for the Chinese-speaking computer vision, generative AI, and multi-modal developer community.

• Translated core philosophy (usability over performance, simple over easy, customizability over abstractions) and 3 primary components (Pipelines, Schedulers, Models).
• Translated installation instructions for PyTorch (pip/conda) and Apple Silicon MPS support.
• Translated quickstart examples for text-to-image inference and building custom diffusion loops from scratch with UNet2D and DDPMScheduler.
• Translated documentation navigation roadmap, contribution guide, and AI Agent development conventions.
• Translated popular tasks table (DDPM, Stable Diffusion, unCLIP, DeepFloyd IF, Kandinsky, ControlNet, InstructPix2Pix, Inpainting, Super Resolution, etc.).
• Translated community ecosystem highlights (TaskMatrix, InvokeAI, InstantID, lama-cleaner, etc.), research credits, and BibTeX citation.
• Added reciprocal language navigation between README.md and README.zh.md.

Before submitting

• Did you read the contributor guideline?
• Did you read our philosophy doc?
• Did you make sure to update the documentation with your changes?

Who can review?

Docs: @stevhliu @sayakpaul

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[github-actions]

Hi @JasonYeYuhe, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.

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[JasonYeYuhe]

Linked tracked translation initiative issue #14711 via Fixes #14711.