Administration - Serverless installation guide includes two istio installation

[adrwong-lora]

Knative should be installed before Istio
Knative itself requires a networking layer, a better way is to tell user to choose istio as the networking layer for Knative instead of treating Knative and Istio as two separate components.

Problem seen
If I install Istio before Knative, Kserve simply doesn't work. It could be due to the unsupported versions or network configured in wrong order.

[yuzisun]

@adrwong-lora Currently we only support Istio as networking layer, why KServe does not work if istio is installed before Knative?