Context

In https://issues.redhat.com/browse/SRVKS-1186 we introduced probing for multiple containers as per request of RHAI. We also need to provide documentation for that feature, which is behind a feature flag.

Documentation contents

The documentation should contain the same information as the upstream docs: https://github.com/knative/docs/pull/5855

One note about this: In the downstream product the feature must be configured using the `KnativeServing` CR with the field `spec.config.features.multi-container-probing` instead of directly in the config-features configmap.

This feature is closely related to multi-container support and probably should be added there: https://docs.openshift.com/serverless/1.31/knative-serving/config-applications/multi-container-support-for-serving.html.