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

[YE (JasonYeYuhe)]

Motivation and Context

1. Why is this change required?
   Semantic Kernel (and its successor Microsoft Agent Framework) is widely adopted by developers and enterprises building LLM agents and multi-agent systems. Providing high-quality, localized Simplified Chinese documentation helps Chinese-speaking developers, researchers, and enterprise users adopt Semantic Kernel more easily.

2. What problem does it solve?
   It eliminates the language barrier and provides accurate, native translations for core concepts (Agents, Multi-Agent Systems, Plugins, Memory, Process Framework, MCP, etc.) directly within the repository.

3. What scenario does it contribute to?
   Documentation accessibility, internationalization, and developer onboarding.

4. If it fixes an open issue, please link to the issue here:
   N/A

Description

• Added a comprehensive 1:1 Simplified Chinese translation of the README (README.zh.md).
• Accurately localized modern enterprise agent concepts (ChatCompletionAgent, Multi-Agent orchestration, Plugin ecosystem, Vector DB integration, Process framework, Ollama/LMStudio/ONNX local deployment).
• Added bidirectional language switcher (English · 简体中文) in both README.md and README.zh.md.
• Added standardized community volunteer maintenance notice with maintainer info (@JasonYeYuhe) and sync timestamp.
• Verified all HTML tags and code blocks formatting.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

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