add drduh yubikey-guide to SSH section.

[AriESQ]

Add drduh yubiky-guide to SSH Section

Adding this repository because it has great in-depth tutorials on how to use Yubikey's with SSH and GPG.

License Assignment

This repository is copyright 2017-2023 Joseph Block under a Attribution-NonCommercial-ShareAlike 4.0 International license.

• I assign copyright to the changes in this PR to Joseph Block per the Attribution-NonCommercial-ShareAlike 4.0 International license.

Types of changes

• New entry
• Typo cleanup
• Text updates

Checklist:

• All new and existing tests pass.
• List entries are sorted alphabetically.
• Any list entries are one line. This makes it easier to keep the sections sorted. Let the markdown renderer handle line breaks, don't try to help it.
• I confirmed that all the links in my PR are valid.

[unixorn]

LGTM

[unixorn]

Thanks for the contribution!