Update Installation instruction for Knative 0.8 release

[ZhiminXiang]

Describe the change you'd like to see
As we will separate cert-manager related components from serving.yaml in our Knative 0.8 release, we need to update the installation instructions accordingly especially the parts related to cert-manager.

Here is the PR in the knative/serving side about the change of yaml generation: knative/serving#4819

[rgregg]

Please include the changes that need to be made to the documentation for this and provide recommendations for how to install cert-manager separately.