Create test cases docs table for `POST /categories/`

## 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 `POST/categories/` API endpoint

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

## Estimation
30 min