Oh I misunderstood sorry.
Then this may be what you are looking for:
If you are using Pandoc citeproc for processing, you can change the CSL style to use a style which place reference in note. You can find some here:
If you want the all citation reference because you won't have a reference part at the end, I would use fullnote style. Here is an example where I used a url for the CSL (but you can download it locally too).
---
title: 'citation in footnote'
output: html_document
bibliography: packages.bib
csl: https://www.zotero.org/styles/chicago-fullnote-bibliography
suppress-bibliography: true
---
```{r, include=FALSE}
knitr::write_bib("knitr", file = "packages.bib")
```
See this citation [@R-knitr]
And we got:

Is this what you are looking for ?
In term of documentation this is mentioned here about changing style: