WCAG: Thumbnail menu should be defined as <nav> element

[Simon-Dirks]

This is from an external accessibility audit of our Mirador 3 install (WCAG 2.2 AA).

Feedback: Marking the thumbnail menu as a nav element would make navigation clearer for screen readers.
WCAG 1.3.1: https://www.w3.org/WAI/WCAG21/Understanding/info-and-relationships.html