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

[JasonYeYuhe]

Description

This PR introduces a comprehensive, high-quality Simplified Chinese translation (README.zh.md) for MuJoCo (Multi-Joint dynamics with Contact) to assist Chinese-speaking researchers, robotics engineers, and reinforcement learning practitioners.

Key Changes

• 1:1 Full Translation (README.zh.md):
  • Full introduction to MuJoCo's C API, preallocated low-level data structures, built-in XML compiler, and OpenGL native interactive viewer (simulate).
  • Interactive Google Colab tutorial links (Introductory, Model Editing with mjspec, multithreaded rollout, LQR humanoid balancing, nonlinear least-squares solver, MJX on JAX, and differentiable physics with analytical gradients).
  • Precompiled binaries, source build instructions, and Python bindings (pip install mujoco).
  • Community support, versioning policy, and contribution channels.
  • Comprehensive first-party and third-party bindings (Python, dm_control/PyMJCF, wasm, Unity, MATLAB Simulink, Swift, Java, Julia, Rust) and model format converters (OpenSim, SDFormat, OBJ, Onshape).
  • Academic citation and licensing disclaimer.
• Bidirectional Navigation: Added language switchers (English · 简体中文) at the top of both README.md and README.zh.md.
• Community Maintenance Notice: Attached volunteer maintenance note (@JasonYeYuhe).

Checklist

• Commits are signed off with DCO (git commit -s).
• All links, badges, Colab badges, tables, and anchor references verified for 1:1 parity and integrity.
• No changes made to source code, simulation engines, or build systems.

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