docs: Add comprehensive development planning documentation

- Created ROADMAP.md with long-term vision and milestones
- Created DEVELOPMENT_PLAN.md with detailed 6-month actionable plan
- Created 开发计划_CN.md (Chinese version of development plan)
- Created CONTRIBUTING.md with contribution guidelines
- Created PROJECT_STATUS.md with comprehensive status tracking
- Updated README.md to reference new planning documents

Co-authored-by: huangyiirene <7665279+huangyiirene@users.noreply.github.com>

Author: Copilot