Documentation: Tutor configuration uses deprecated YAML files

[regisb]

In this XBlock readme, the recommended approach is to create a Tutor plugin as a YAML file. But plugins as YAML files are no longer the recommended approach to create Tutor plugins. Instead, plugin developers are encouraged to create either single-file Python modules or full-fledged Python packages: https://docs.tutor.edly.io/tutorials/plugin.html