docs: expand API endpoints documentation

[ankitasahu83964]

Summary

• Added detailed API endpoint documentation in docs/API.md
• Documented HTTP methods, authentication requirements, request examples, success responses, and common errors
• Added documentation for authentication, products, cart, checkout, orders, wishlist, addresses, and promotions
• Linked the detailed API documentation from README.md

Testing

• npm run check:syntax ✅

Related Issue

Closes #1731

[hydra-maintainer]

🔍 Quality Gate Report

✅ All quality gates passed!

Status	Check	Details
✅	Linked Issue	PR description references a closing issue ✅

[vercel]

@ankitasahu83964 is attempting to deploy a commit to the Bhuvansh's projects Team on Vercel.

A member of the Team first needs to authorize it.

[hydra-maintainer]

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[hydra-maintainer]

🔴 PR Health Score: 30/100

This PR's health score is below the 75/100 threshold for a healthy label.

Status	Signal
❌	Test coverage
✅	Linked issue
✅	PR description (≥50 chars)
❌	DCO sign-off
⬜ 0 approval(s)	Approvals
❌	Diff size < 400 lines

Improving these signals will help reviewers engage faster and raise your score. 💪

[hydra-maintainer]

🔍 Quality Gate Report

✅ All quality gates passed!

Status	Check	Details
✅	Linked Issue	PR description references a closing issue ✅

[hydra-maintainer]

🤖 AI Code Review

🔴 Score: 50/100 | comment

AI review unavailable at this time.

---

Automated AI review — a human maintainer will also review.

[hydra-maintainer]

🔴 PR Health Score: 30/100

This PR's health score is below the 75/100 threshold for a healthy label.

Status	Signal
❌	Test coverage
✅	Linked issue
✅	PR description (≥50 chars)
❌	DCO sign-off
⬜ 0 approval(s)	Approvals
❌	Diff size < 400 lines

Improving these signals will help reviewers engage faster and raise your score. 💪

[ankitasahu83964]

Hi maintainers, I ran the backend test suite locally after completing the documentation changes.

The backend suite currently reports pre-existing failures unrelated to this documentation PR:

• mergeScarRegression.test.js
• cartBulkSelection.test.js
• wishlistReads.test.js

The failures are in existing application/test expectations around product stock bindings, global cart handlers, and wishlist product filters. The PR itself only changes docs/API.md and README.md.

The syntax check passes successfully with npm run check:syntax.