Perform manual testing and report POST /login API

## Description

As a tester,
I need to test POST /login API,
so that I can provide a test report on it.

You should create a markdown file inside `/docs/manual_tests` with the following content:
- Scenario tested step by step with dummy data you used (e.g.: username and password)
- Report it as:
```
* _Test1 Description_
_Screenshot/gif_:
_Expected Result_:
_Actual Result_:
* _Test2 Description_
_Screenshot/gif_:
_Expected Result_:
_Actual Result_:
...
```
If you find a test that the expected result does not match the actual result then create an issue (following [Bug Report template](https://github.com/anitab-org/mentorship-backend/blob/develop/.github/ISSUE_TEMPLATE/bug_report.md)), and mention it in your markdown file.
- Make sure to test at least:
- 1 positive case (e.g.: login with a registered user correct username and password)
- 1 negative case (e.g.: logging not registered user)

**Resources:**
- Deployed Backend: https://mentorship-backend-temp.herokuapp.com/
- Quality Assurance test cases: [/docs/quality-assurance-test-cases.md#login](https://github.com/anitab-org/mentorship-backend/blob/develop/docs/quality-assurance-test-cases.md#login)

## Acceptance Criteria
### Update [Required]
- [ ] Test POST /login API
- [ ] Create a markdown file under `/docs/manual_tests` with the content mentioned above (at least 2 tests).

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

## Estimation
3 hours