doc: Design for certificate pruning
    
    This describes how the certificate pruning capability of PKI
    introduced in v11.3.0 will be integrated into IPA, primarily for
    ACME.
    
    Related: https://pagure.io/freeipa/issue/9294
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>