Rename and update description of AspNetCoreRuntime package

We should normalize the name of this package i.e. Microsoft.AspNetCoreRuntime.* or Microsoft.AspNetCore.Runtime.SiteExtension.*

We should also update the description or add a readme so users know what the package does (and so we don't forget 😆)
> It adds the 2 main shared runtimes to app service as well as the latest native IIS bits. This isn't super interesting when App Service has the latest bits already, but during previews/nightlies it makes it easier to use App Service.

https://github.com/dotnet/aspnetcore/blob/79ef5e329b1e31c3775a1977798253cc8f7da6cc/src/SiteExtensions/Runtime/Microsoft.AspNetCore.Runtime.SiteExtension.pkgproj#L11