[Accessibility] Vignette plots lack alt= attributes

[MichaelChirico]

We have a tool that flags images that lack an alt= attribute. See https://help.siteimprove.com/support/solutions/articles/80000863904-accessibility-image-alt-text-best-practices -- these are essential for conveying the image contents to users on screen readers.

You can add these to your vignettes by using the recent-ish fig.alt chunk option in knitr; see https://www.rstudio.com/blog/knitr-fig-alt/.

There are a lot of issues, so I'll just name the vignettes:

• Edges
• Layouts
• Nodes
• tidygraph

If you'd like more details I can provide, but I'm assuming it's basically clear enough what plots are at issue.