Create guide on using SSH keys with GitHub

[kelly-sovacool]

Changes

Classic PATs are going away on March 1st. We've been using them to authenticate with GitHub when running git operations on our laptops. We could switch to fine-grained tokens, but they will have to expire every 90 days and you have to create a new fine-grained token for each account. Instead, we should recommend everyone switch to using SSH keys for authentication.

This PR creates:

• an overview of the GitHub authentication options
• instructions on setting up ssh keys
• instructions on using fine-grained tokens i.e. for Code Ocean
• warnings about classic tokens and updated links throughout

Issues

resolves #35
resolves #59

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• Update CHANGELOG.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/
• Render the website locally with quarto render and view the documents changed in this PR to ensure they are rendered correctly.