Duplicated guidance on setting the download directory

[Robinlovelace]

We should do it once and do it in one place I think.

Currently there are at least 3 .qmd files with this info, should we just cover it properly in one place (the convert vignette seems most appropriate at the moment) and link to that one place?

https://github.com/search?q=repo%3ARobinlovelace%2Fspanishoddata+%22directory%22+path%3Aqmd&type=code