Better alt-text for publication teaser

[Dev-Lan]

Teaser figures in the detailed publication page have all have an alt-text of " screenshot". This is ok-ish for pages where it is a screenshot of a tool like Loon/Aardvark (https://vdl.sci.utah.edu/publications/2021_vis_loon/, https://vdl.sci.utah.edu/publications/2024_vis_aardvark/).

However, this doesn't work as well for pubs that don't have a screenshot. E.g. the alt-text for this cartoon is "misleading screenshot" https://vdl.sci.utah.edu/publications/2023_chi_misleading/

Instead, I think we should add an attribute to the publication markdown so we can write custom alt-text for the teaser image. If that field is not populated we can fall back to the current approach.