Include default logging dictConfig in documentation

[Andrew Badr]

When people want to customize Django's logging, my guess is that they'll most often want to start by changing something about the default logging setup. Toward this end, it would be helpful if the Django dogs included a dictConfig for the default logging setup. Instead, the docs *describe* the default settings (​https://docs.djangoproject.com/en/2.1/topics/logging/#default-logging-configuration), and give dictConfigs for several other possible configurations for logging (​https://docs.djangoproject.com/en/2.1/topics/logging/#configuring-logging).