Add HostBuilder to 3.0 migration docs (#10718)
Fixes #10618, Fixes #10711
[Internal review URL](https://review.docs.microsoft.com/en-us/aspnet/core/migration/22-to-30?view=aspnetcore-3.0&branch=pr-en-us-10718&tabs=visual-studio#generic-host-replaces-webhostbuilder)