Add docs on probes

[psschwei]

Describe the change you'd like to see We have some info on what types of probes the user can add to the Services here, but it'd be nice to have a doc (possibly at something like https://knative.dev/docs/serving/services/user-probes) that describe what types of probes users can set (readiness/liveness), which ones they can't right now (startup, grpc), and an example of how to configure one.

Additional context Came up in a slack discussion