make accessible to screen readers

[cnell-usgs]

• provide alt text that includes the actual data shown
• alt text for each of the images
• read the screen readers section of this notebook for additional suggestions https://observablehq.com/@9a849ed5a351d187/adventures-in-d3-accessibility

[cnell-usgs]

Looks like this has largely been addressed with #50

Adding notes from the PR:

• Get the focus to switch to the Dialog when it is open, so that the user can use keyboard navigation within the dialog.
• See: Definitely important for accessibility, per Getting started with Vue accessibility article
• See: Briefly explored a few things per this, this, and this, but they didn't work and I didn't dig further b/c I wanted to get the other changes up. I think the key is the 'ref' piece and triggering focus when the dialog is opened.
• Test and make modifications, as needed. Unclear how the d3 titles will be handled.