Fix docs link to package tests docs

[jgreener64]

Since #52102 these links to the Pkg.jl docs are dead. This should point them to the new location in the Julia docs.

There is a reference in CONTRIBUTING.md that should change as well, but maybe not until a Julia version with these changes is released:

julia/CONTRIBUTING.md

Line 17 in 3e4b386

- Library feature requests are generally not accepted on this issue tracker. New libraries should be developed as [packages](https://julialang.github.io/Pkg.jl/v1/creating-packages/). Discuss ideas for libraries at the [Julia Discourse forum](https://discourse.julialang.org). Doing so will often lead to pointers to existing projects and bring together collaborators with common interests.