Add comprehensive developer documentation

- Created DEVELOPER.md with complete technical guide
- Covers prerequisites, project structure, and architecture
- Documents build process (local and CI/CD)
- Explains version management and release process
- Details technical limitations (WPF, no AOT, no trimming)
- Provides debugging tips and common issues
- Includes contributing guidelines
- Links to relevant documentation and resources

This provides everything a developer needs to understand, build,
and contribute to the project.

Author: shanselman