Document how to use impersonation when using Windows auth and SignalR from a Blazor Server app

[danroth27]

When a user wants to use SignalR in a Blazor Server app and authenticate using Windows auth, they likely need to use impersonation to pick up the users identity from the server process. @BrennanConroy and @halter73 shared how to do this here: dotnet/aspnetcore#25000 (comment). @guardrex Can you take a look at adding this guidance to the Blazor docs?