Once work is completed in #808, insert these descriptive captions as an alt-text attribute of the figure.
Need to determine syntax, then implement in xml files
captions/alt text work has been completed - see text in https://github.com/cu-mkp/edition-webpages/blob/master/docs/resources/figure-index.md
Addition to markup has yet to be done. Consultation with @tcatapano and @NickLaiacona and @caro27 will be required