Document logging configuration schema

[DamianEdwards]

The logging system is now (since 2.0) integrated directly with the configuration system, such that it can be configured with a set schema, via the various config providers (JSON file, environment vars, cmd-line args, etc.). All of the ASP.NET Core project templates (except Empty) include a default configuration in their appSettings.json and appSettings.Production.json but I couldn't find any docs on how to change it, or how it interacts with configuration from other providers, .e.g. how to set from env vars, precedence of configuration when set in multiple providers (common in places like Azure App Service), etc.

@glennc