docs(zh-CN): add Simplified Chinese README (#33)

Add README.zh-CN.md as a Mainland Simplified Chinese translation and update the English and Traditional Chinese language navigation so all three maintained READMEs link to each other.

Preserve the canonical English structure, commands, URLs, versions, numeric claims, inline code, and Star History embed attributes. Apply Mainland terminology to the reviewed localization points while keeping shared technical vocabulary consistent across the existing translations.

Verification:
- compared headings, tables, executable Bash blocks, URLs, versions, numeric claims, inline code, local links, and embed attributes against README.md
- ran git diff --check
- simulated the merge onto main 2b24be6; only README.md, README.zh-CN.md, and README.zh-TW.md change
- scanned the PR range with gitleaks; no leaks found
- completed a clean Codex review on current head 18c38eaa
- resolved all review threads

Co-authored-by: 周州 <54611681+javaht@users.noreply.github.com>