convert vignette rmd code chunks to plain R chunks to prevent eval

[tanho63]

Closes #301.

I ran find-replace on the following:

```{r eval = FALSE}
```{r, eval = FALSE}
```{r} # where a setup chunk set eval = FALSE at the top

and replaced with

```r

which converts it to plain syntax-highlighted markdown rather than an executable code chunk. This should hopefully prevent evaluation of vignette code that is not meant to be executed

[jennybc]

I'm reviewing this and I think I want to only modify the one vignette that was called out in #301. It had caused some pain for me and for an external party, whereas we haven't had pain from any other source.

I think all these other vignettes and articles ... are best left as they are. I suspect having eval = FALSE R chunks is superior, in terms of formatting said R code, which is also worth considering. I think one day I might do a broad converstion to .qmd, which will present an obvious opportunity to make other changes.

I'll probably just push my desired changes into this PR and then merge. Thanks for getting the ball rolling!