docs: add Chinese (Simplified) translations for core documentation

[OLDBAI213]

This PR adds Chinese (Simplified) translations for the core ISNAD documentation, addressing bounty issue #4.

Translated files

• README.zh-CN.md — Root README translation
• docs/README.zh-CN.md — Documentation index / Getting started guide
• docs/what-is-isnad.zh-CN.md — "What is ISNAD?" overview
• scanner/README.zh-CN.md — Scanner CLI usage documentation

Translation guidelines followed

• All code blocks, commands, URLs, and technical terms kept in English
• Descriptions, explanations, and comments translated to natural Chinese
• Same markdown structure and formatting maintained
• Badges and shields.io links preserved as-is
• Language switcher links added at the top of each translated file

Closes #4

[vercel]

@OLDBAI213 is attempting to deploy a commit to the Rapi's projects Team on Vercel.

A member of the Team first needs to authorize it.