Add a new WebApplication entry to Fundamentals => Host

https://github.com/dotnet/AspNetCore.Docs/pull/25092#issuecomment-1051295846:

> I think we should add a new entry under "Host" next to the existing "Generic Host" and "Web Host" entries called "WebApplication". The migration doc by itself isn't that helpful to people unfamiliar with the generic host already. I don't want to force people to learn generic host concepts first in order to understand the simplified WebApplication. I do understand not wanting to have 3 host sections with a bunch of duplicate content though.