docs: remote: Replace broken link to article about 'ssh-agent'

docs: remote: Replace broken link to article about 'ssh-agent'

The documentation about remote access to libvirt was pointing to a blog
post about usage of the 'ssh-agent' to avoid being asked for passwords.

The blog/host is now unfortunately defunct thus the link is dead.

Replace the link by the official man page of the 'ssh-agent' program.

This convenietnly removes the last non-'https' link on our web.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>