Lack of documentation on queue-proxy probing

**Describe the change you'd like to see**
Comprehensive documentation on queue-proxy probing controls and behaviors.

**Additional context**
Readiness probing, and specifically aggressive probing, is part of the queue-proxy sidecar. Some services are slower to start than others, may have backend dependencies such as a database, etc. However, there is really no documentation on how to configure the queue-proxy probing behavior in the documentation.

e.g. As of today there are conflicting [notes / comments](https://github.com/knative/serving/issues/10838#issue-814745385) that seem to describe the same thing about disabling aggressive probing, with [opposite instructions](https://github.com/knative-sandbox/container-freezer#container-freezer) in another knative project. One says to set `periodSeconds>0` while the other says to set `periodSeconds=0` to disable aggressive probing.

Start by reading the queue-proxy probing context in the linked Knative issue 10838 and the container-freezer notes referenced in this issue. Document the probing controls and behavior, including the authoritative setting for disabling aggressive probing and how readiness probing handles slow services or backend dependencies.

Written by the indexing model from the issue text.