Add migration guide and implementation summary for Branch SDK modernization

- Introduced a comprehensive migration guide detailing the transition from legacy to modern Branch SDK APIs, ensuring 100% backward compatibility.
- Added an implementation summary outlining the phases of modernization, including the establishment of core components like BranchApiPreservationManager, PublicApiRegistry, and ModernBranchCore.
- Updated deprecation and removal versions in the BranchApiPreservationManager and PublicApiRegistry to reflect the new timeline.
- Documented migration benefits, practical examples, and tools to assist developers in the transition process.

Author: wpinho-branch