Create a single root logger, rather than a root logger configuration

If a file sink is added to a shared `LoggerConfiguration` like the original demo uses, then `.CreateLogger()` is called multiple times, the individual loggers will compete for use of the shared file.

The idiomatic way to do this is generally to create a root logger, then use `ForContext()` to create more specialized loggers off of it. This will aggregate events correctly into a single log file.