Update repository documentation

[thomasnemer]

Summary

Updates repository documentation to reflect the current state of the project and removes outdated references.

Fixes #13

Changes Made

README.md Updates

• ✅ Removed ThemeParser references (doesn't exist) and updated to AdvancedTheme
• ✅ Updated theming section to reflect actual APIs (Color, ThemeManager, AdvancedTheme)
• ✅ Moved Roadmap to reference GitHub issues as authoritative source
• ✅ Clarified that adapters are example implementations in examples/, not core framework
• ✅ Fixed all code examples to use correct Scene-based architecture
• ✅ Verified all code examples are accurate and compilable
• ✅ Made Scene-based architecture prominent throughout

CONTRIBUTING.md (New File)

Created comprehensive contributing guide with:

• Development environment requirements (C++20, CMake, Ninja, mise)
• Building instructions (mise build, mise demo, manual CMake)
• Development workflow (mise format, mise test, mise demo)
• Code style guidelines (CRTP, header-only, relative coordinates, component architecture)
• Testing guidelines
• Pull request process
• Git hooks policy with strong warnings against bypassing

Files Changed

• README.md - 105 insertions, 74 deletions
• CONTRIBUTING.md - 513 lines (new file)

Testing

• All documentation changes reviewed for accuracy
• Code examples verified against actual codebase
• No references to non-existent features
• Clear distinction between core framework and examples

Acceptance Criteria

• README.md has no references to non-existent features (ThemeParser)
• README.md theming section reflects actual API (Color, ThemeManager, AdvancedTheme)
• Roadmap moved to reference GitHub issues
• Adapter positioning clarified (examples, not core)
• CONTRIBUTING.md created with all sections
• Clear mise workflow documentation
• Code style guidelines documented
• Git hooks policy documented (never bypass)
• All code examples verified to be accurate