add alt text so images aren't duplicated in html
add alt text so images aren't duplicated in html

workaround proposed in https://github.com/spatialaudio/nbsphinx/issues/162

jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets