Add info on switching from HTTPS to SSH

Why is this change being made?

Adds information on how, after using the single script setup method, you can switch your cloned www-gitlab-com from HTTPS to SSH.

I've proposed this after, when going through this step as part of my onboarding, I found it confusing that when I tried to push a handbook edit from my local repo, I was asked for my personal access token, instead of my SSH key or passphrase. After speaking to IT help, they gave me a command to run that would switch the repo from HTTPS to SSH. I thought it would be useful to state this in the handbook.

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Author Checklist

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