adapt CA creation docs for pro

[daringer]

Generally the CA creation docs (https://docs.nitrokey.com/pro/linux/certificate-authority.html) also work with the Pro, but the commands differ due to different auth mechanisms, e.g., the initial key creation needs to look like this:

pkcs11-tool -l --keypairgen --key-type rsa:2048 --label root --so-pin 12345678 --login-type so

we need to define how we handle these (smaller) document differences, best case we would have a generic document which describes CA overall and then something like "nitrokey pro -> generate keypair" would be linked and a separate smaller article