refactor: simplify project - remove community docs and custom theme, use English metadata

Remove unnecessary files and simplify configuration:

- Remove CODE_OF_CONDUCT.md and CONTRIBUTING.md (community governance)
- Remove src/privacy.md (privacy policy)
- Remove custom theme files (404.html, cookie-consent, custom CSS/JS, PWA files)
- Update book.toml description to English
- Update theme/head.hbs to English description with minimal SEO
- Remove 'Tài liệu Khác' section from SUMMARY.md

Keep core improvements:
- Async/Await guide (src/advanced/async/)
- Unsafe Rust guide (src/advanced/unsafe/)
- Image alt text fixes
- GitHub Actions caching
- .editorconfig
- External link checking enabled

Author: duyetbot