Add guidance for configuring multi-factor authentication

[tobyhodges]

If this pull request addresses an open issue on the repository, please add 'Closes #NN' below, where NN is the issue number.

Fixes #89

Please briefly summarise the changes made in the pull request, and the reason(s) for making these changes.

This adds a section to the Setup instructions about creating a GitHub account, including some guidance for learners about configuring multi-factor authentication. We are outsourcing most of the configuration instructions to GitHub themselves, as the interface and process may change relatively often.

Please proofread this and correct any mistakes before merging!

If any relevant discussions have taken place elsewhere, please provide links to these.

This change was discussed with Maintainers of other Git lessons in The Carpentries, and further by the https://github.com/orgs/swcarpentry/teams/curriculum-advisors.

[github-actions]

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 md5sum.txt |  2 +-
 setup.md   | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

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⏱️ Updated at 2023-09-25 09:42:47 +0000

[MGomezN]

Thanks @tobyhodges! It looks good to me, plus, this modification has already been merged in the sister lesson git-novice