add guidance for configuring 2FA on GitHub accounts

[tobyhodges]

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

Fixes #922

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.

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 @swcarpentry/curriculum-advisors.

This PR replaces #973

[github-actions]

Thank you!

Thank you for your pull request 😃

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🔍 Inspect the changes: https://github.com/swcarpentry/git-novice/compare/md-outputs..md-outputs-PR-974

The following changes were observed in the rendered markdown documents:

 md5sum.txt |  2 +-
 setup.md   | 36 ++++++++++++++++++++++++++++++++++--
 2 files changed, 35 insertions(+), 3 deletions(-)

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⏱️ Updated at 2023-09-26 14:57:28 +0000

[coatless]

LGTM! 👍

[kekoziar]

@tobyhodges, thanks for incorporating the feedback =)