Update docs and `knative-routing` sample to use Gateway/VirtualService

[ZhiminXiang]

/area networking
/kind doc

Since we already check in PR #1228 to migrate to Istio v1alpha3 Gateway / VirtualService. We need to update samples and docs accordingly.

[tcnghia]

Thanks, @ZhiminXiang. I tried updating all samples in knative/serving in #1228 but @trisberg pointed out that I missed one named knative-routing.

[tcnghia]

I think knative-routing doc also copied to https://github.com/knative/docs/tree/master/serving/samples