This commit addresses Phase 0.4 roadmap priorities:

## CI/CD Improvements
- Remove `--ignore-missing-imports` from mypy in CI for stricter type checking
- Add GitHub Actions workflow for automatic documentation deployment
- Documentation now auto-deploys to GitHub Pages on docs changes

## Documentation Infrastructure
- Add custom CSS styling (docs/stylesheets/extra.css) for enhanced readability
  - Custom color scheme for GenesisGraph brand
  - Priority and disclosure level badges
  - Improved tables, code blocks, and search results
- Add custom JavaScript (docs/javascripts/extra.js) for improved UX
  - Version badge in header
  - Scroll-to-top button
  - Enhanced search highlighting
  - Keyboard shortcuts (Ctrl+K for search)
  - Edit on GitHub link

## Contributing Guide Enhancements
- Add "Current Priorities" section linking to v0.3.0 → v1.0 roadmap
- Document Phase 0.4 focus areas (test coverage, API docs, type checking)
- Add guidance on high-value contributions
- Include issue labels for contributor discovery

## Impact
- Stricter type checking catches more errors before merge
- Documentation site ready for deployment
- Contributors have clear guidance on priorities
- Better documentation UX for users

Related to roadmap items:
- Phase 0.4: Foundation & Quality (4-6 weeks)
- Increase test coverage to 90%+
- Create API documentation site
- Tighten type checking