Add comprehensive permission issues guide (#580)

- Created PERMISSION_ISSUES_GUIDE.md with detailed solutions for permission problems
- Based on thorough analysis of issues #558, #556, #555, #549, #496, #501, #492, #420
- Includes platform-specific solutions (NixOS, macOS, Windows, Synology)
- Documents critical Docker version requirements (20.x+ recommended)
- Highlights rootless image as recommended solution for permission issues
- Added link to guide in README.md troubleshooting section

Key findings documented:
- Most "Operation not permitted" errors resolved by updating Docker
- Rootless Docker requires using actual Docker UID (e.g., 100999) not 845
- Interrupted chown operations cause inconsistent file ownership
- Rootless images avoid most permission issues entirely

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: fank