git module - add section about ssh key

[valosekj]

Students at our hub are encountering permission issues when pushing their local branch to GitHub because they have not configured an SSH key. I refer them to our intranet lab tutorial, but we could add the details directly to the module.

[htwangtw]

Thanks for raising this issue!

Your lab tutorial is a great summary for the github docs! It would be great to update this to the module.