Update docs to remove asp-prerender-webpack-config

[SteveSandersonMS]

The newer templates don't use asp-prerender-webpack-config any more, because it's simpler and more powerful to prebuild a prerendering bundle. We should update the setup tutorial in the docs to match this.

Eventually we'll remove asp-prerender-webpack-config and LoadViaWebpack entirely, but they will stay for a while for backward compatibility.