Documentation cross references between packages

[c42f]

Given that JuliaHub can now build and serve most package documentation, it would be really neat if packages could link to symbols which they import from other packages with @ref and have that generate a link to the other package's documentation.

This would greatly improve the user experience of the Julia ecosystem documentation as a whole.

This could be done with some customization of the way that @ref is looked up by Documenter - see some related Documenter issues:
JuliaDocs/Documenter.jl#1343
JuliaDocs/Documenter.jl#319

Documenter configuration currently isn't declarative which makes this more difficult than it might ideally be, but perhaps that can also be fixed eventually (see JuliaDocs/Documenter.jl#1087 (comment)).