Marek Lewandowski3ad25227c6Docs: Added documentation for the [`fontColor`](https://ckeditor.com/docs/ckeditor5/latest/api/module_core_editor_editorconfig-EditorConfig.html#member-fontColor) and [`fontBackgroundColor`](https://ckeditor.com/docs/ckeditor5/latest/api/module_core_editor_editorconfig-EditorConfig.html#member-fontBackgroundColor) configuration options in [`EditorConfig`](https://ckeditor.com/docs/ckeditor5/latest/api/module_core_editor_editorconfig-EditorConfig.html) interface. Closes #40.