feature request: popovers for footnotes / non-RFD references

[iliana]

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confidential info from RFDs.

References to RFDs have a nice popover with information about the RFD in question:

It would be nice to have a popover for other references, too, as well as footnotes. I suspect they won't look as polished as the RFD ones (I don't think there's a standard format for references we use), but I think it's pretty important to be able to read the text and click inner links without needing to scroll all the way to the bottom.