docs on how to set up Tilt port-forwarding with Traefik

In the #tilt channel, Meerkat asks:

I'm using tilt on a cluster that already has Traefik deployed on it, as in tilt has no knowledge of traefik while running.
But I need to do the tilt equivalent of this command:

kubectl port-forward --namespace traefik $(kubectl get pods --namespace traefik --selector "app.kubernetes.io/name=traefik" --output=name) 8000:8000

Currently, Tilt's port-forwarding logic isn't customizable enough to support something like this, since Tilt doesn't even watch pods that it doesn't deploy.

You can wire this together with

local_resource(serve_cmd=[shell script here])

to run the port-forwarder as a "local server"

I'm not totally sure what the right answer is here, though Traefik is becoming popular enough that we should have documentation on the recommended solution for something like this.

Start from the issue's kubectl port-forward command and the local_resource(serve_cmd=[shell script here]) example, then review how Tilt handles port-forwarding for resources it does deploy. Document the recommended way to connect Tilt to an externally managed Traefik instance, including what a working port-forward setup should look like.