Support lightbox experience for viewing full-size images

[mfisher87]

Also noted here: https://github.com/jupyter-book/mystmd/issues/1809

Proposal

Like Quarto, support displaying large images in the page at a scaled-down size, with the ability to click them to view the full-size image.

```{figure} https://example.com/my-image.png
:lightbox:

My caption
```