Show "indexes" for citations

[algo74]

Being able to identify citations by their reference numbers in the paper is very helpful for reading:

This may be hard to implement in general. However, simply showing indexes helps in cases where all the paper's citations are imported.
A simple hack would be to change
const label = citation.target.getLabel(); to const label = (index+1).toString() + ". " + citation.target.getLabel(); in line 241 in citationsBox.jsx, but a better approach could be to make it an option, perhaps individual for each document.