Demo 2nd non-collection example for virtualization with an item provider delegate

[guardrex]

I think GetEmployeesAsync for the items provider delegate example returns a collection. DataRow is a potential 2nd example for a use case that isn't a collection. Whether or not the example is shown, let's extend the current guidance to indicate that Virtualize expects a generic ICollection<T>; so if the items aren't a collection, the delegate approach is the right way to go. Example and discussion is at https://github.com/dotnet/aspnetcore/issues/37875#issuecomment-970692620.

---

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 0f4ab8b4-4d2e-9e1b-793e-dbb17f6d694d
• Version Independent ID: 26ed2425-63ec-e206-edcb-e51f17edb844
• Content: ASP.NET Core Razor component virtualization
• Content Source: aspnetcore/blazor/components/virtualization.md
• Product: aspnet-core
• Technology: aspnetcore-blazor
• GitHub Login: @guardrex
• Microsoft Alias: riande