link to ambient installer docs from main installer docs

[craigbox]

curl -L https://istio.io/downloadIstio has output taking people to https://istio.io/latest/docs/setup/install/, and this page will be high in Google results.

We also have our Getting Started guide at https://istio.io/latest/docs/setup/getting-started/, linked from "Try Istio" on every page.

While ambient docs are in a different hierarchy, we should link to the ambient install guide/s from these pages.