end-to-end ingress instructions

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## Background
https://linkerd.io/2/tasks/exposing-dashboard/ describes exposing `linkerd dashboard` via an ingress resource. Configuration of the ingress-controller (nginx or traefik) is left up to the user.

## Proposal

Update the docs to include an end-to-end working example of exposing `linkerd dashboard` via ingress, including ingress controller configs, service resources, and support for websockets.

These configs could even live in the https://github.com/linkerd/linkerd-examples repo, allowing the instructions to simply be:

```bash
linkerd install | kubectl apply -f -
curl https://raw.githubusercontent.com/linkerd/linkerd-examples/master/ingress/nginx.yml
open localhost.linkerd # or similar, prefer not requiring /etc/hosts modification
```

## Related issues

- linkerd/website#211
- linkerd/linkerd2#2143

Start with the existing exposing-dashboard task at linkerd.io/2/tasks/exposing-dashboard/ and review the related issues linkerd/website#211 and linkerd/linkerd2#2143. Determine whether the example belongs in the website or linkerd-examples, then provide a reproducible ingress-controller setup with service resources and websocket support; done means the dashboard works through ingress without manual controller configuration or an /etc/hosts edit.

Written by the indexing model from the issue text.