Package-level doc

[jennybc]

I'm talking about foo-package.html, for the package foo. Right now this is a bit of an orphan wrt the function reference page. It is neither automatically included, but neither do you get the usual Warning: Topics missing from index: .... Should one of those be true? The html is getting generated, but isn't accessible by any series of clicks, I believe.

[hadley]

It's sort of hidden on purpose - I think we're moving towards this file being largely autogenerated (e.g. r-lib/roxygen2#669) so I'd rather not promote it.