user alerts for use_pkgdown()

[jayhesselberth]

use_pkgdown() is recommended in the pkgdown README so many are probably using. more feedback might reduce surprises later on (e.g., not realizing docs/ was git-ignored.)

r-lib/pkgdown#2087

[jayhesselberth]

We might also consider prompting the user to ask whether they intend to use docs/ directory or the gh-pages branch on GitHub Pages. If the former, then we shouldn't git-ignore docs/

[jennybc]

use_pkgdown() already emits all of these messages:

devtools::load_all("~/rrr/usethis")
#> ℹ Loading usethis

create_local_package()
#> ✔ Setting active project to '/private/tmp/Rtmpdm53vZ/testpkg36bfc716496'
use_pkgdown()
#> ✔ Adding '^_pkgdown\\.yml$', '^docs$', '^pkgdown$' to '.Rbuildignore'
#> ✔ Adding 'docs' to '.gitignore'
#> ✔ Writing '_pkgdown.yml'
#> • Edit '_pkgdown.yml'

^{Created on 2023-06-23 with reprex v2.0.2.9000}

I suspect that a better baseline recommendation in pkgdown is usethis::use_pkgdown_github_pages().