Copy the contents of `docs.R` to a vignette

[eitsupi]

I think this document should be a vignette. https://github.com/pola-rs/r-polars/blob/9a09cb48ef38efbfc05cf25ef371856f49d53d2b/R/docs.R

[etiennebacher]

Oh wow I never came across this file. I'm not sure it should be a vignette, there is a lot of different stuff in there: how R objects are treated in Rust, details on Floats and Ints, on how Python dicts are equivalent to lists, etc. There are also some things that I don't think are interesting enough to be in a vignette, like the analogy with base plot and ggplot2 at the end.

I think we could use some parts of it in additional vignettes. For example, at some point I would like to have a vignette that clearly explains how missing data are handled (like when are they broadcast and when are they ignored?). We could use the two paragraphs about "Missingness" and "Nulls" in the linked file for that.