docs: add contribution guide

[li041]

Summary

• Add a comprehensive CONTRIBUTING.md guide for hvisor contributors.
• Document repository responsibilities and cross-repository change coordination.
• Define quality, testing, integration, ABI compatibility, AI-assisted development, and licensing requirements.
• Link the contribution guide from both English and Chinese README files.

Motivation

hvisor changes can affect multiple architectures, boards, hypervisor interfaces, management tools, and documentation repositories. The new guide establishes a consistent contribution process and makes testing, compatibility, and cross-repository responsibilities explicit for contributors and maintainers.

Verification

No functional behavior, ABI, boot flow, or platform support was modified.

Risks and Limitations

• This is a documentation-only change with no expected runtime or ABI impact.
• The documented repository links, workflows, and testing requirements may need updates if the project structure or release process changes.

[agicy]

LGTM.