Document how to use hardware secure keys with github-keygen

[dolmen]

github-keygen doesn't limit the values for the -t option, so security keys should work out of the box with the existing releases of github-keygen if the OpenSSH client has security key support. We just need to improve documentation to tell users how to do.

Related:
https://docs.github.com/en/github/authenticating-to-github/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent#generating-a-new-ssh-key-for-a-hardware-security-key
https://github.blog/2021-05-10-security-keys-supported-ssh-git-operations/