Advocate for .rds rather than .RData in FAQ (R users)

Here https://computo-journal.org/blog/2023-07-04-what-reproducibility/#intermediate-results-storage we suggest to store intermediate results in .RData files. 

I think we should rather advocate for the .rds format in order to avoid the side effects of the `load` function (ie possibly overriding elements of the global environment without notice).
