docs: Add migration guide and Google repo tool comparison

Closes #8

Add two major documentation sections to enhance user adoption:

**Migration Guide for Existing Dependency Trees:**
- Bottom-up migration strategy starting from penultimate level
- Step-by-step PowerShell example with 6-repository dependency tree
- Semantic versioning compliance (v4.6.0 for compatible changes, not v5.0.0)
- Proper shared dependency handling (DatabaseUtils and CommonControls in single locations)
- Visual dependency tree diagrams showing before/after migration states
- Directory structure visualization demonstrating shared vs specific dependencies
- PowerShell testing and validation commands
- Migration tips and best practices

**LsiGitCheckout vs Google's Repo Tool Comparison:**
- Feature comparison tables covering configuration, platform support, and dependency management
- Use case recommendations and target user profiles
- Migration considerations in both directions
- Balanced analysis of strengths and limitations for each tool
- Technical requirements and scalability comparisons

**Integration with existing documentation:**
- Updated Table of Contents with new sections
- Cross-references to existing features (API Compatibility, Post-Checkout Scripts)
- Enhanced Traditional Package Managers section with post-checkout integration examples
- Consistent PowerShell-native approach throughout

This addresses user adoption barriers by providing clear migration paths and
tool selection guidance for teams with existing multi-repository projects.

Author: AndreaV-Lsi