docs: reorganize READMEs and extract backend protocol reference

[zengyi1001]

Summary

The READMEs mix installation and configuration instructions with detailed backend protocol content. This change reorganizes the English and Chinese READMEs around setup, configuration, troubleshooting, and contribution, and moves the protocol reference into dedicated bilingual documents.

• Add docs/protocol.md and docs/protocol_zh_CN.md covering QR login, messaging, lifecycle notifications, and CDN media handling.
• Distinguish client behavior, field definitions, and integration guidance; clarify thumbnail handling, plaintext image downloads, message examples, and per-endpoint error handling against the current source.
• Add documentation navigation and explain the existing difference between the runtime version guard and the npm peer dependency requirement.

Only Markdown files change.

Validation

• Relative links, JSON examples, code fences, bilingual heading/example structures, and git diff --check passed.
• npm ci --ignore-scripts and local quality checks (format, lint, typecheck, unit tests, build) passed during the README reorganization.
• npm test passed: 403 tests; branch coverage 90.04%.
• Subsequent protocol-only revisions passed document validation. Existing lint warnings and dependency audit findings remain unchanged.