Add OAuth2 Provider Documentation

This PR adds comprehensive documentation for the experimental OAuth2 Provider feature, which allows Coder to function as an OAuth2 authorization server. The documentation covers:

• Feature overview and experimental status warning
• Setup requirements and enabling the feature
• Methods for creating OAuth2 applications (UI and API)
• Integration patterns including standard OAuth2 and PKCE flows
• Discovery endpoints and token management
• Testing and development guidance
• Troubleshooting common issues
• Security considerations and current limitations

The documentation is marked as experimental and includes appropriate warnings about production usage.