[DOC] Make sure we are clear about th PKCS#8 certificate format requirement

[scholzj]

As discussed in https://github.com/orgs/strimzi/discussions/11751#discussioncomment-14243883, it seems that after the move from PKCS12 to PEM certificate stores, we require the certificate to be in the PKCS#8 format (while iot seems that while the intentional support was originally for PKCS#8, we through OpenSSL supported PKCS#1 as well).

We should make sure this is clearly documented in the docs. In the long term, this should apply to the user certificates in Connect/MM2/Bridge ... but likely also to CAs and custom listener server certificates.