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

[YE (JasonYeYuhe)]

What does this PR do?

This PR adds a complete 1:1 Simplified Chinese translation (README.zh.md) of the main documentation and a PyPI-safe language switcher at the top of README.md to improve accessibility for Chinese-speaking LLM researchers and developers.

Changes

• Added README.zh.md: Complete, natural Simplified Chinese translation of README.md covering:
  • News and overview of LLMLingua, LongLLMLingua, LLMLingua-2, and SecurityLingua (plus MInference, RetrievalAttention, and SCBench links)
  • Quick start installation and Python API usage for PromptCompressor across LLMLingua, LongLLMLingua, LLMLingua-2, and SecurityLingua
  • Structured prompt compression with <llmlingua> tags, custom data collection/training links, FAQ, contributing guidelines, and BibTeX citations
• Updated README.md: Added a centered language navigation link (English · 简体中文) using a PyPI-safe absolute GitHub URL (https://github.com/microsoft/LLMLingua/blob/main/README.zh.md) so links work both on GitHub and on the PyPI llmlingua package page.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Was this discussed/approved via a Github issue? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

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Anyone in the community is free to review the PR once the tests have passed.

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