Add a complete Gateway creation example to operator README

Both upstream Istio and OSSM2 start with an ingress (and/or egress) gateway that allows users to deploy the Bookinfo application without further actions. Because the Sail/Istio operator does not start with an ingress gateway, users must create one. While we point to upstream documentation for creating a Gateway, it may not be a straightforward task for many users. You will either have to install helm, enable Gateway API or put together a Gateway Injection deployment template. This is a further challenge for OpenShift users, as the upstream Gateway Injection docs will not work out of the box on OCP (I had to refer to the OSSM2 docs to get this working).] 

Thus, we should provide an example config that allows a user to copy/paste or better yet - a yaml that will let the user execute an "oc/kubectl apply -f ..." command as they do for the rest of Bookinfo's commands.

I attached the Gateway Injection config that I used, which includes a Service to expose the Gateway. This should be reviewed by others though to see if it is appropriate - it should be made as simple as possible.

Note: The future product doc will require similar guidance and a quick start for Gateway configuration, so the content will be useful for OSSM3 onboarding doc as well.