Add detailed Q&A guide for Azure DevOps to GitHub migration using GitHub Enterprise Importer

- Comprehensive overview of migration scope, including elements migrated and special cases (Git LFS, Go packages).
- Outline of general effort and team requirements for successful migration.
- Pre-migration repository analysis strategies and recommendations.
- Step-by-step migration process, including trial migrations and measuring success.
- Guidance on post-migration tasks, including mannequin reclaim and repository permissions.
- Appendices with CLI quick reference, platform size limits, required PAT scopes, and official documentation links.

Author: CalinL