Create Quality Assurance table for List All Tasks 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 "List All Tasks" API to the docs in the /docs folder.

Refer to #757, if the following task has not been performed. 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 List All Tasks API.
### Enhancement to Update [Optional]
- [ ] 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