Create Quality Assurance table for Delete Task API

## Description
As a developer,
I need to know what are the test cases for a certain API,
so that I can write new unit tests.

As a tester,
I need to know what are the test cases for a certain API,
so that I can perform manual tests on the API.

The task is to come up and add a quality assurance table for "Delete Tasks" API to the docs in the /docs folder.

Additionally, add all tasks' quality assurance tables under one heading. Currently, the Update task is under heading "Mentorship Relation" and Create Tasks under heading "Tasks". I suggest make the subheading Tasks under Mentorship Relation and add all Tasks' API under it.

#### Note: This issue is labeled for Open Source Hack.
I can assign this issue to you on September 1st as the official opening date for Open Source Hack. Please free to check out other issues in the meantime. Program website: https://anitab-org.github.io/events/open-source-hack/

## Mocks
As done under [docs/quality-assurance-test-cases.md](https://github.com/anitab-org/mentorship-backend/blob/develop/docs/quality-assurance-test-cases.md)


## Acceptance Criteria
### Update [Required]
- [ ] Add test cases for Delete Task API.
- [ ] Add all tasks' quality assurance tables under one heading.

## Definition of Done
- [ ] All of the required items are completed.
- [ ] Approval by 2 mentors.

## Estimation
[4+] hours