docs(avalonia): add custom gutter implementation notes and solutions

Comprehensive documentation of the custom gutter challenge:
- Root cause: AvaloniaEdit 0.10.12 vs Avalonia 11.3.x version conflict
- Four potential solutions analyzed (adorner, canvas, TextMate, fork)
- Recommended approach: Adorner-based overlay
- Implementation guide with code sketches
- Testing strategy and performance considerations
- Future enhancements roadmap

Provides clear path forward for Phase 4.3+ gutter implementation.

Author: lookbusy1344