Explain how to configure logback

## Description
The topic [logging](https://www.dita-ot.org/dev/topics/logging.html) explains how to configure FOP logging with logback. It would be great to find an example here of how to add a custom logback.xml file to the classpath.

> To enable debug logging, modify the config/logback.xml file or add your own logback.xml to the classpath with a higher priority to override the default settings. For more information, see the [Logback configuration documentation](https://logback.qos.ch/manual/configuration.html).