Add more info on hover on cross-references in gitbook/bs4_book?

[maelle]

I am reading the R Markdown cookbook that includes many cross-references to other pages. I think it'd be neat if when hovering on one of these cross-references, I got a preview of the page (title, first sentence). E.g. at the top of https://bookdown.org/yihui/rmarkdown-cookbook/indent-text.html I'd like to be able to remember what section 4.12 is without having to click (even if the URL itself that I see on hover is a bit informative).

I think this issue is related to #689.

I don't know how this would work, maybe with html5 data attributes. The feature is also similar to bs4_book() making footnotes available on hover.

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[cderv]

I think this is a great idea!

Some references of the tool that can be used for this: distill is using tippy and popover for some tooltip / popover (https://github.com/rstudio/distill/commit/3a83b0288d3ddc4ae4977dcfa48e8b713d1d02e6)

[cderv]

And with bootstrap this is included: https://getbootstrap.com/docs/4.0/components/tooltips/ (which is what bs4_book() uses)