Update guidance about nginx-ingress

Created by: kevinwojo

Follow-up from https://github.com/sourcegraph/deploy-sourcegraph/pull/4128 Update documentation to position ourselves to not include the nginx ingress controller in our reference deployment and shift this to the customer's responsibility.

The ingress controller is more implementation-specific ... each cloud provider has their own implementation for example. We may want to encourage customers to choose one that works for them rather being too prescriptive.