Setup instructions could be clearer

We've been using the instructions for setting up users in our organization, and have found they usually need some additional explaining to make them work.

It seems like all the information is there, but there's some back and forth involved, skipping some steps, etc.

I think the sections "GSA Standards" and "Creating a GitHub Account" could benefit from some additional review and maybe even user-testing.

One suggestion is that two major paths may be:
- user who has a personal GitHub account and will begin using that for GSA work (more steps and options)
- user who does not have a personal GitHub account and will be creating a brand new one (fewer steps and options)