Better document Pomerium as standalone solution.

[alexfornuto]

Many new Pomerium users begin by setting us up with Traefik or Ingress as the major traffic controller, using us in forward auth mode. Pomerium is a first class proxy and works best independently of competing services.

We want to better highlight that our Proxy service is build on Envoy, why it's production ready, and possibly why it's not a great idea to use us just for forwardAuth.

[desimone]

We could borrow some "backed-by-envoy" messaging here from https://www.getambassador.io/

or other like istio