Is there any documentation regarding creating a custom editor button?

I want to provide users with the ability to reference player stats from an API by adding a custom button that takes the variable of username.

You have two options. Your first option is to create a basic editor button right from the AdminCP, which allows you to account for most basic formatting needs.

If you need to create a more complex editor button, you can refer to CKEditor's documentation directly: https://docs.ckeditor.com/ckeditor4/latest/