[manual] Mention `import Foo as _`

[ericphanson]

For loading packages just for their side-effects. Discussed here: https://discourse.julialang.org/t/is-there-tooling-to-mark-unused-imports/129638/7

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[ericphanson]

if we merge https://github.com/JuliaLang/julia/pull/58681 we should update the docs to recommend using Foo: _ over using Foo: Foo as _ on recent Julia's.