docs: add English version of contribution guide

Translate the complete Chinese contribution guide to English, including:
- Overview, Code of Conduct, and Governance sections
- Roles and Responsibilities (Users, Contributors, Maintainers)
- Getting Started (GitHub account registration, CLA signing)
- Submit Issue and Submit PR common processes
- Code Contribution workflow
- Documentation Contribution (website quick edit and full Git workflow)
- Component Adaptation Contribution (detailed 4-step process)
- FAQ section

The English version maintains the same structure and content as the Chinese
version, providing bilingual support for the IvorySQL community.

Author: jiaoshuntian