Consider replacing screenshot images with knitr::include_url

[cmorganl]

Rather than having to potentially update multiple screenshots for this project, we could use the function include_url from the knitr package. Example usage is below:

knitr::include_url("https://www.uniprot.org/uniprot/?query=%22particulate+methane+monooxygenase%22&sort=score", height = "400px")

A couple of issues exist:

• knitr::include_url is still being developed and I've seen many issues caused by this function. Patches are in the development branch on the knitr repository but not on CRAN. So at the very least, we will need to wait until the functionality is stable.
• Intermittent server outages that coincide with bookdown builds would result in empty images

I thought it would be an interesting enhancement so I just wanted to log it for the future. Nothing needs to be changed immediately.