docs: add English documentation for README, CONTRIBUTING, and DOWNLOAD_GUIDE

[HARSHRAO729]

Summary

This PR introduces complete English translations across all major project documentation files, along with bidirectional language switcher links (简体中文 | English) to enhance accessibility for international users and contributors.

Key Additions & Changes:

1. README:
   • Added README_EN.md with full English translation.
   • Added language toggle at the top of README.md & README_EN.md.
2. Contribution Guide:
   • Added .github/CONTRIBUTING_EN.md with full English translation.
   • Added language toggle at the top of .github/CONTRIBUTING.md & .github/CONTRIBUTING_EN.md.
3. Download Guide:
   • Added .github/DOWNLOAD_GUIDE_EN.md with full English translation.
   • Added language toggle at the top of .github/DOWNLOAD_GUIDE.md & .github/DOWNLOAD_GUIDE_EN.md.
4. Cross-linking:
   • Updated README_EN.md to reference .github/CONTRIBUTING_EN.md and .github/DOWNLOAD_GUIDE_EN.md.

Type of Change

• docs: Documentation update

Verification

• Verified cross-links and language toggles between Chinese and English documentation files.
• Ensured compliance with conventional commit conventions (@commitlint/config-conventional).