docs: add comprehensive getting started guide

- Add detailed PackageURL development setup documentation
- Explain PURL specification and 20+ supported ecosystems
- Include parser/builder development workflows
- Document type-specific normalization and validation
- Add Result<T> pattern usage examples
- Explain 100% coverage requirement
- Include troubleshooting section for common issues
- Add security considerations for JSON serialization
- Provide quick reference tables and commands
- Link to existing docs (api-reference, builder-pattern, etc.)

This reduces new contributor onboarding time and provides a single
source of truth for PURL development setup and common workflows.

Author: jdalton