docs: add comprehensive V3 migration guide

- Document V2 to V3 migration checklist
- Add code migration examples (Python, JavaScript, Bash)
- Document CI/CD workflow migration patterns
- Add fork migration guide with step-by-step instructions
- Document backward compatibility and deprecation timeline
- Add troubleshooting section with common issues
- Include local testing instructions and FAQ
- Document V3 principles and benefits

Author: SckyzO