[ELUSoC'26] Create Comprehensive API Documentation Guide

[KGFCH2]

Problem

There is no centralized guide documenting backend API endpoints, payload requirements, or response schemas, which makes it harder for new contributors to get started.

Current Behavior

Developers must inspect the router and controller code to understand API structures.

Why This Improvement Is Needed

Clear API documentation improves the onboarding process for new contributors and simplifies frontend-backend integration.

Proposed Solution

Create docs/API_DOCUMENTATION.md detailing all backend endpoints, authentication flows, and JSON schemas.

Expected Outcome

A complete, easy-to-read API reference guide is added to the project repository.

[github-actions]

Thank you for raising this issue!

We'll review it as soon as possible. We truly appreciate your contributions! ✨

Meanwhile make sure you've visited the README.md, CONTRIBUTING.md, and CODE_OF_CONDUCT.md before creating a PR for this. Also, please do NOT create a PR until this issue has been assigned to you. 😊