Add information on authorising SSH keys

[wmartin-gss]

Problem with 'Setup GitHub' (https://user-guidance.analytical-platform.service.justice.gov.uk/github/set-up-github.html)

I was helping a colleague onboard to managing an AP webapp, but they had trouble cloning the repo; it turns out that their SSH key needed to be authorised to access repos in the ministryofjustice organisation (where our webapp lives). It would be really helpful to flag this in the guidance, as we spent a while troubleshooting what was going wrong.