API page: https://api.drupal.org/api/drupal/core%21core.api.php/group/config_api/8
This page is supposed to give you the basics of the Configuration API.
It was apparently never updated after the getEditable() requirement went in (if you want to alter config, you need to call getEditable() on the config object because the default config object you get is not editable).
So the examples involving changing/saving configuration in the Simple Configuration section of this page need to be updated to use code similar to what you see on
https://www.drupal.org/node/1809490
Should be a fairly straightforward Novice project I think...