docs: reorganize documentation and add comprehensive SemVer mode coverage

Fixes #12

- Move migration content to separate docs/migration_guide.md
  * "Migrating Existing Dependency Trees" for both Agnostic and SemVer modes
  * "Handling Shared Dependencies Version Changes" with complete SemVer scenarios
  * Identical dependency tree examples for easy mode comparison

- Move tool comparisons to separate docs/comparison_guide.md
  * "LsiGitCheckout vs Google's Repo Tool"
  * "LsiGitCheckout vs Traditional Package Managers"
  * Enhanced post-checkout script integration examples

- Streamline main README.md for better user experience
  * Focus on core functionality and getting started quickly
  * Add comprehensive "Choosing Between Dependency Resolution Modes" section
  * Clear advantages/disadvantages and decision criteria for SemVer vs Agnostic
  * Enhanced dependency resolution modes documentation with SemVer details

- Improve documentation organization and navigation
  * Clear sectioning distinguishing Agnostic vs SemVer content throughout
  * Updated cross-references to new document structure
  * Consistent terminology and formatting across all documentation
  * Proper table of contents with mode-specific sections

Key improvements:
- SemVer mode now has complete coverage with identical scenarios as Agnostic mode
- Simplified SemVer compatible updates showing zero-configuration advantage
- Clear guidance for teams choosing between dependency resolution modes
- Modular documentation structure supporting easier maintenance
- Enhanced user experience with focused main README and detailed guides

Author: AndreaV-Lsi