Add Swagger API Documentation for Backend #126

Description:
Integrate Swagger (via swagger-ui-express and swagger-jsdoc) to document the backend API endpoints.
This will make it easier for contributors and external developers to understand, test, and consume the API.

Why this is needed:

• Provides clear and interactive API documentation
• Improves collaboration between backend and frontend contributors
• Helps maintain consistency and discoverability of endpoints
• Useful for future public API exposure or external integrations

Acceptance Criteria:

• Swagger dependencies (swagger-ui-express, swagger-jsdoc) installed
• Swagger configuration file (swagger.js or similar) created
• Routes documented for all existing endpoints (GET /mentors, GET /mentors/:id, etc.)
• Documentation available at /api-docs endpoint
• Add short section in README.md explaining how to access and use the Swagger UI