Use html_vignette instead of html_document

[llrs]

As explained here this would reduce the size of the vignettes. Which some users noticed on the mailing list some time ago.

[vjcitn]

Are there other implications of this change? It would be nice to have an example of effects in a reasonably complex vignette example -- say one that uses DT::datatable and perhaps some htmlwidgets

[llrs]

I am not aware of any other implications but I haven't test it, if there is interest I might look for some complex vignettes (not sure how to find them)