Make it possible to preview built vignettes during CRAN submission

[jennybc]

Inspired by revising the vignettes chapter of R Packages.

One thing that has come into focus is that we want to discourage the keeping of built vignettes inside a source package, meaning below inst/doc/. The need to do this has been greatly reduced by pkgdown (esp. + GitHub Actions + GitHub Pages), which offers a way to make built vignettes readily available for GitHub-only packages, for dev versions, etc.

But to take this position, you really do have to think about and address all the reasons that people have historically wanted to keep these built products around. One motivation is that sometimes you want to look over the built vignettes you're sending to CRAN, since they are, in fact, what CRAN will ultimately distribute. This is especially true if you've had to do anything tricky with eval.

Currently release() and submit_cran() are monolithic, i.e. there's no way to pause between package build and submission, right around here:

devtools/R/release.R

Line 226 in aa3f88b

built_path <- pkgbuild::build(pkg$path, tempdir(), manual = TRUE, args = args)

We could have a yesno() to browse the built vignettes and, if yes, to approve them and proceed with submission.