Documentation on "fail closed" egress setup

[ihcsim]

Provide documentation on how to set up egress gateway to enable "fail closed" egress calls based on https://github.com/grampelberg/k8s-egress.

[wmorgan]

I would do two things here: I would add a Features stub similar to what we do with Ingress, and have it point to a Tasks walkthrough that describes the set in the k8s-egress repo. (Which maybe we want to move into our ancient linkerd-examples repo? Don't know if we've ever used that for 2.x stuff but this seems appropriate.)