docs: add centralized REST API reference to README

[Vaishnavi10706]

📌 Description

This PR adds a centralized REST API Reference section to the README to improve developer onboarding and backend integration.

Previously, contributors had to inspect backend route files manually to discover available endpoints and understand authentication requirements. This update provides a single source of truth for the project's API documentation.

🔗 Related Issue

Closes #1422

🛠 Changes Made

• Added a new REST API Reference section to README.md

• Documented available API endpoints for:

  • Authentication
  • Tasks
  • Routines
  • Analytics

• Included HTTP methods, endpoint paths, authentication requirements, and endpoint descriptions

• Added JWT Authorization header usage example

• Verified documented routes against existing backend route files

• Kept changes limited to documentation only with no backend or frontend code modifications

✅ Checklist

• Code runs locally
• Followed project structure
• No console errors
• Properly tested changes
• Linked the issue

🚀 Notes for Reviewers

• This PR only updates README.md.
• No application logic, API behavior, or project configuration was modified.
• Documentation was aligned with the existing backend route structure to improve maintainability and contributor experience.