docs: restore missing POST /suggestions API reference section

[Vaishnavi10706]

Description

This PR fixes a formatting issue in the README.md API Reference.

Changes made

• Restored the missing ### POST /suggestions/ section heading.
• Added the endpoint description.
• Inserted the missing opening ```json code fence.
• Restored proper Markdown formatting and JSON syntax highlighting for the example response.
• No API behavior or project code was modified.

Related Issue

Fixes #1709

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read [CONTRIBUTING.md](../CONTRIBUTING.md)
• My branch is up to date with main
• I have run pytest -v and all tests pass (not applicable – documentation-only change)
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues) (not applicable)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

Not applicable.

Test evidence

Documentation-only change. No code or tests were modified.