Create test cases docs table for `GET /categories/{id}/sections`

## Description
As a developer or tester,
I need to know some test cases examples,
so that I can have an idea of how I can test the app.

Creating test cases will help everyone understand how to use the API.

References:
- [Quality Assurance test cases](https://github.com/anitab-org/mentorship-backend/blob/cf6df094e4fef735e135674e4d5697ded5060d7d/docs/quality-assurance-test-cases.md) for mentorship-backend project
- Check out the API Swagger definition here: https://stem-diverse-tv.herokuapp.com/

## Acceptance Criteria
### Update [Required]
- [ ] Create quality assurance documentation on `/docs` folder.
- [ ] Create test cases documentation table for `GET /categories/{id}/sections` API endpoint.

## Definition of Done
- [ ] All of the required items are completed.
- [ ] Approval by 1 mentor.

## Estimation
30 mins