Add OAuth2/OIDC integration preparation documentation

- Add OAUTH2-README.md: Overview and navigation guide
- Add OAUTH2-QUICK-START.md: 15-minute setup guide with code examples
- Add OAUTH2-OIDC-INTEGRATION-PREP.md: Complete 60-page implementation guide

Documentation covers:
- 6-phase implementation plan (6 weeks)
- Integration with OBP-OIDC provider
- Reference implementation from OBP-Portal
- Complete code examples for all components
- Testing strategy and deployment guide
- Backward compatibility with OAuth 1.0a

Author: simonredfern