Create test cases docs table for `POST /users/register`

## 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 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 /users/register` API endpoint
### Definition of Done
- [ ] All of the required items are completed.
- [ ] Approval by 1 mentor.
### Estimation
30 min