Rework "importing Katello root CA certificate" to make fit for all builds

[maximiliankolb]

The procedure https://github.com/theforeman/foreman-documentation/blob/master/guides/common/modules/proc_importing-the-katello-root-ca-certificate.adoc only works for Katello. In non-Katello setups the /pub location (/var/www/html/pub in the CLI procedure) doesn't exist so we need to instruct users with an alternative way to get this for foreman-el and foreman-deb.

Originally posted by @ekohl in https://github.com/theforeman/foreman-documentation/pull/4063#discussion_r2235536427

[ekohl]

I've reworded it to be more descriptive of what really needs to be done.