use_quillt_pkgdown() should add the vignette infrastructure too

[cderv]

I think from rstudio/rticles#412 as some files were missing (rstudio/rticles@8f81237) that use_quillt_pkgdown() is not complete.

As we are creating the vignette from a template

quillt/R/templates.R

Lines 93 to 97 in f6c6729

	# Add vignette used as Get Started.
	usethis::ui_info("Adding package named vignette for Get Started section.")
	use_template("vignette-intro.Rmd",
	intro_rmd <- file.path("vignettes", paste0(data$Package, ".Rmd")),
	data = data, ignore = FALSE, open = FALSE)

we don't use usethis::use_vignette(), which leads to missing part in the package when no vignettes are already there.

This should be improved.