API doc: validate generated reference
    
    Extend 'makeapi --validate' to validate API Reference files too. If
    differences are found between the generated and stored docs the
    validation fails. This command is executed in our Azure pipelines, so
    every time a developer opens a PR but forgets to update the API
    Reference, the CI will fail.
    
    Fixes: https://pagure.io/freeipa/issue/9287
    Signed-off-by: Antonio Torres <antorres@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>