Add docs on GitHub team authentication

[j0nnyr0berts]

• This PR updates the documentation to reflect the addition
  of GitHub team based authentication to OAuthenticator

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[manics]

This looks good to me (preview at https://zero-to-jupyterhub--2349.org.readthedocs.build/en/2349/administrator/authentication.html).

FYI there was a long complicated discussion in the past on GitHub scopes: #687

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[consideRatio]

Thank you @j0nnyr0berts!