Consider migrating /inst templates to /vignettes

[dgkf]

I was just poking around in the code today and noticed that there are vignette-like documents in /inst. There can be good reasons to keep them here, but to my naive eye it seemed like they'd be a good fit for vignettes.

One of the benefits of moving them to /vignettes is that they are tested and re-built as part of your R CMD check checks and more easily discoverable since they follow a convention for how this type of content is usually distributed.

There's a great chapter on vignettes in the R Packages book