[DOCS] Update the oss.gg Readme as complete setup guide

[jobenjada]

Is your feature request related to a problem? Please describe.

The oss.gg readme file needs to serve as a step by step setup guide.

Currently, you need follow the readme first and then find this video: https://youtu.be/KkKwqcw-h-k

Describe the solution you'd like

Please write up a detailed step by step guide for a developer to setup oss.gg with all necessary GitHub apps (OAuth and oss-gg-testapp)

If you have any questions or get stuck, ping @jobenjada

Describe alternatives you've considered

No response

Additional context

No response

[nik132-eng]

/assign

[oss-gg]

You already have an open issue assigned to you here. Once that's closed or unassigned, only then we recommend you to take up more.

[jobenjada]

updated