ingress: update docs on how to setup linkerd with Contour

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### Problem
While running through the instructions on [how to setup Contour to work with Linkerd](https://linkerd.io/2/tasks/using-ingress/#contour), I ran into the following error:

```bash
$ linkerd inject https://projectcontour.io/quickstart/contour.yaml | kubectl apply -f -

Error transforming resources: cannot enable mTLS when automountServiceAccountToken set to "false"
```

This happens because the change introduced in https://github.com/linkerd/linkerd2/pull/4145 causes `inject` to error out when a resource has `automountServiceAccountToken: false`. The existing docs do not account for this change in behavior of `inject` CLI.

### Solution

We could re-write the `inject` step such that users would have to first manually set `autmountServiceAccountToken` to `"true"`, or we could host the corrected version of the YAML so that users could directly plug in the URL into `inject`.

We could also add a note letting the users know that if their CLI version is older that ` Linkerd2-edge-20.4.1` (when the change was introduced), they can perform injection first, and then enable `automountServiceAccountToken`

Start with the Contour instructions at linkerd.io/2/tasks/using-ingress/#contour and reproduce the documented linkerd inject command against the referenced Contour YAML. Update the instructions to handle automountServiceAccountToken: false and verify that the revised steps work with the affected Linkerd CLI versions.

Written by the indexing model from the issue text.