Add Microsoft.AspNetCore.App to dotnet docs repo

[Rick-Anderson]

Update the Metapackages sections to include Microsoft.AspNetCore.App

[guardrex]

We may need to move a blurb from the Kestrel topic over here. It pertains to the reasoning behind having to add an explicit package reference to the Libuv transport package to 2.1 apps that use the App package to avoid a breaking change for 2.1. See the discussion on #6184. Once it's over here, we can link it in the Kestrel topic instead of maintaining the details there. cc/ @natemcmaster

[EDIT] ... or the migrations topic for 2.1. That would also probably work. There's no current issue for a 2.1 migrations topic AFAICT.

[Rick-Anderson]

This has been updated in the donet docs repo