docs: Add short HOWTO doc on Yubikey usage

[jku]

We should document how Yubikeys are used. Things to mention

• the required library (and that we've only tested with yubicos library)
• how to create the signing key
• PKCS11LIB env var required by the signer

A minimal document would be good

Some material exists in these issues/comments:
#472 (comment)
#229

[jku]

Turns out Yubikey Manager may be the most reasonable way to create a signing key:
https://github.com/jku/repository-playground/blob/main/playground/yubikey-manager.png

(I spent an hour fighting with yubico-piv-tool again yesterday, IMO it's buggy and complicated)

[lukpueh]

👍 I too used Yubikey Manager to initialize the Yubikey. But I can't remember an option to create the key.

[jku]

Go to Applications -> PIV -> Digital Signatures and click Generate. Then you get a wizard to choose keytypes etc