Add comprehensive developer documentation

- Create DEVELOPMENT.md with complete development guide
- Add docs/ARCHITECTURE.md with system architecture details
- Add docs/RPC.md with RPC integration documentation
- Add docs/PAYMENT_FLOW.md with payment processing details
- Add docs/README.md as documentation index
- Update README.md to reference new documentation

Co-authored-by: deverickapollo <5827364+deverickapollo@users.noreply.github.com>

Author: Copilot