Add sample yaml files for custom configuration

[sampaccoud]

Purpose

Although the edxapp Docker image includes default settings that work out of the box, we need some information about how to customize such settings via the yaml files.

Proposal

We don't want to add example yaml files in the /config directories because we would think they are necessary and they would uselessly override the default values that have already set in the .py files.

We propose to include an example of yaml file configuration as a snippet in documentation.