This is a follow-up to https://issues.redhat.com/browse/RHDHBUGS-45

Right now, using the Operator, a customer can reference an external certificate as a Secret in their CR, like so:

spec: 
  application: 
    route: 
      enabled: true
      host: my-rhdh.apps.example.com
      tls: 
        externalCertificateSecretName: my-rhdh-tls-cert

The `spec.application.route.tls.externalCertificateSecretName` field is then used by the Operator to create a route with this external certificate : https://github.com/janus-idp/operator/blob/main/pkg/model/route.go#L75-L79

However, as depicted in https://docs.openshift.com/container-platform/4.16/networking/routes/secured-routes.html#nw-ingress-route-secret-load-external-cert_secured-routes, Securing route with external certificates in TLS secrets is a Technology Preview feature only. As such, it requires enabling the `RouteExternalCertificate` Feature Gate in OpenShift.

I wonder if we should not call this out explicitly in our product docs.

Note that this does not affect the Helm Chart, since I didn't find this ability exposed in the Route object created by the Chart: https://github.com/redhat-developer/rhdh-chart/blob/main/charts/backstage/templates/route.yaml