Include documentation for the use of Postman to test GraphQL queries

**Is your feature request related to a problem? Please describe.**
While I was working on a new feature, I found testing and understanding the queries and parameters to be hard as we can only change them in code, run them then check the network tab.

**Describe the solution you'd like**
I decided to use Postman to test and try out different queries

Here is a simple example of testing and running using Postman



We will be able to test graphql queries first without having to touch any code.

**Describe alternatives you've considered**
Maybe another solution would be to use [octokit-playground](https://github.com/graphql/graphql-playground) library as we are already using octokit to run the queries

[Github Explorer](https://docs.github.com/en/graphql/overview/explorer) works great too as there is no need for any extra authentication.

**Additional context**
Perhaps having more information for testing and learning might make it easier for other contributors and encourage implementing more complex features using graphql.

Perhaps we can consider
- Postman with a collection of all the queries built-in with variables (reduce the need of having setup queries every time we want to use it)
- Github explorer, easy to use however, every query requires setting up relevant variables

Start by reviewing the Postman examples and the repository's GraphQL query usage described in the issue, then locate the current contributor or user documentation. Done means contributors can follow documented steps to test GraphQL queries with Postman, including variables, without changing application code; compare the alternatives mentioned, such as GitHub's GraphQL Explorer.

Written by the indexing model from the issue text.