support responsive images via <picture> element

[maxheld83]

when it comes to HTML outputs of bookdown, this blogdown issue also applies here: rstudio/blogdown#46

Except in bookdown, HUGO and Hugo themes CSS/HTML aren't in the picture, so I am wondering whether it might be easier to do this.

This is, admittedly, quite a long shot.

To explain why I'm even bringing this up: I sometimes have gitbook pages with a lot of quite detailed plots (scatter plots), and it is beginning to show in the load times.
Obviously, I don't want needlessly long load times, but I also like to provide high resolution plots for people on big / HiDPI screens etc.

[jemus42]

I mentioned it in the other issue on blogdown, but my comment is still valid I guess:

You can use knitr hooks to customize the plot output HTML and it seems to work okay, especially if you don't need fancy gallery plugins like lightbox to work as well.

[yihui]

I'll leave it to yourself to implement via the plot hook, since I feel this is too much work for me, and practically I don't have the time.

[github-actions]

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