docs: make Chinese README the default

[MatrixA]

Summary

Make the Chinese README the default repository landing page while preserving the English version under docs.

Changes

• switch the root README.md to the Chinese documentation and add the Bilibili demo link
• add docs/README.en.md to preserve the English documentation
• update localized README navigation and fix LICENSE links under docs

Checklist

• cargo fmt -- --check passes
• cargo clippy -- -D warnings passes
• cargo test passes
• New detection methods include appropriate confidence tiers
• Documentation updated (if applicable)