Document using logging config from existing configuration object

[captainsafia]

The current logging docs in Blazor show how you can add logging config using an in-memory property or the ILoggingBuild API methods.

We don't showcase how you can read the logging config included in your appsettings.json file.

To do this, the user needs to invoke builder.Logging.AddConfiguration(builder.Configuration.GetSection("Logging")) in their startup logic.

More context at dotnet/aspnetcore#20969.

[guardrex]

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/logging/?view=aspnetcore-3.1#create-logs-in-blazor

<PackageReference Include="Microsoft.Extensions.Logging.Configuration" Version="3.1.4" />

using Microsoft.Extensions.Logging;