Document on all hops encrypted amongst internal and external services in Knative

[nak3]

What is changing? (Please include as many details as possible.)

Once knative/serving#13005 was merged, we should add the documentation about the feature.
(I am going to create the draft docs so please allow me to write the detail later.)

How will this impact our users?

Currently there are no docs about internal TLS. When we document it, users can refer to the usage and so on.

In what release will this take happen (to the best of your knowledge)?

• Serivng 1.6

Context

Link to associated PRs or issues from other repos here.

• Support config to deploy internal certificates automatically serving#13005
• All hops encrypted amongst internal and external services in Knative serving#11906

[abrennan89]

@nak3 any update on this one?