I want to add a custom button to the TinyMCE editor. But the documentation about this feature is non-existsten as you can see here: https://docs.joomla.org/Plugin/Events/Editor#onCustomEditorButton
Do you have any idea how this works or other sources of a useful documentation?
What you've currently link to in your question is a plugin event for a custom editor button, not for the actual creation of one.
To create a custom button, check the following directory:
ROOT/plugins/editors-xtd/
where you will see a few core extension.
You can copy any of these and start writing your code inside the onDisplay() function
