git hook should check if pkgdown is up to date

[dmurdoch]

If I add a new function to my package but forget to add it to _pkgdown.yml, then the pkgdown build on Github will fail. It would be nice if one of the git hooks wouid warn me about this.

I'm not sure if the warning should be a pre-commit or pre-push warning.

Currently I don't have a pre-push hook, but I have a pre-commit hook that checks that README.md has been updated after README.Rmd was changed. I believe that hook was installed by usethis::use_readme_rmd(). I don't think anything in usethis installs a pre-push hook, so it would probably be easiest to do that, so you don't need to worry about merging the two hooks. I think usethis::use_pkgdown() would be the place to put it.

A test could be that if NAMESPACE is newer than docs/reference/index.html, it could suggest running pkgdown::build_site() or maybe pkgdown::build_reference(), which should update the index file.