Specify where to obtain cert for standalone hammer

[aneta-petrova]

non-blocking: Installing standalone hammer also has this as a prerequisite https://github.com/theforeman/foreman-documentation/blob/cddb86f4df06266d752c7d50771031cc0097e9d9/guides/common/modules/proc_installing-standalone-hammer.adoc?plain=1#L11

Now that we figured out a way how to actually get it, should we mention it there as well?

Originally posted by @adamruzicka in https://github.com/theforeman/foreman-documentation/pull/4425#discussion_r2478160463

[ekohl]

On my phone now, but I have some recollection Hammer has a sort of TOFU to get the certificate. https://github.com/theforeman/hammer-cli/blob/master/lib/hammer_cli/ca_cert_fetcher.rb is at least a source that implies it can just fetch it itself. Could be worth checking the source if it can be optimized using this new endpoint.