The bar that you are referring to is the Markdown Editor called “Quill”. It is a separate open-source project that has been incorporated into v11. So it is not Frappe/ERPNext code, it is not a simple matter of “adding a button”.
If you are up to it, there is a workaround here that replaces the Quill Editor with the Jodit Editor: Significant Quill WYSIWYG Editor problems (v11) - #17 by MichaelPinkowski