List all the possible values for toolbar items

[mcavazotti]

📝 Provide a description of requested docs changes

I couldn't find any where a full list of all the possible items for the toolbar. It would help new users (such as I) to have it in the EditorConfig docs: https://ckeditor.com/docs/ckeditor5/latest/api/module_core_editor_editorconfig-EditorConfig.html#member-toolbar