Add detailed API documentation, error handling, and security best pra…

jeremah
2025-01-22T18:27:06Z

…ctices to `backend/README.md`.

* **API Documentation**
- Add detailed descriptions of each endpoint under Admin Routes.

* **Error Handling**
- Add sections for custom error types, error handling in routes, controllers, and models.

* **Security Best Practices**
- Add sections for input validation and sanitization, authentication and authorization, rate limiting, and logging.

Add `DOCUMENTATION.md` with project architecture, design patterns, and coding standards.

Update `.github/copilot-instructions.md` to reference new documentation files.