Document and test cross-package doc linking

[jayhesselberth]

From: https://github.com/r-lib/pkgdown/issues/715

Surprisingly this works for cross-package documentation

...
- title: Visualization & Display
  desc: Graphics functions and print formats
  contents:
  - tf::plot.tf
  - geom_spaghetti
  - gglasagna
  - geom_capellini
  - tf::print.tf
  - type_sum.tf
....  

[hadley]

That's definitely tested, but I think we might've forgotten to document it.

[maelle]

It's the last item in https://pkgdown.r-lib.org/reference/build_reference.html#topic-matching (easy to miss for sure :sweat_smile: )

[maelle]

Ok to close, or is there a place where it should be added? the autolinking vignette?