Add Lightbox support for figures

I've used the standard Antora UI for the IntelliJ plugin documentation. I found that the current UI doesn't support the "lightbox" effect: As a user I want to enlarge a figure in the document by clicking on the image. I can then close this image again by clicking on "X" in the upper right corner or clicking on the image again.

See here for an example (click on the image to trigger it):

https://intellij-asciidoc-plugin.ahus1.de/docs/users-guide/quick-start.html

An all-in-one handlebar template (HTML, CSS, JavaScript): https://github.com/ahus1/intellij-asciidoc-plugin.ahus1.de/blob/master/_antora/supplemental_ui/partials/lightbox.hbs

This feature requires JavaScript. If the user doesn't have JavaScript enabled, it degrades gracefully (that is: no lightbox support, and also no changed cursor). Advanced users can use the "open image in new tab" option when right-clicking on the image.

Please let me know if this would be a good addition for the default UI theme, and if there is something that should be ammended/changed. One addition would probably be to add a check that the figure doesn't have a hyperlink that would otherwise be shadowed. Based on the comments and the example above I'm happy to prepare a pull request.

Screenshot: