Add docs for npm auth setup

[bluwy]

This is only a quick draft about setting up npm auth for users.

We could consider leaving auth setup completely to users (especially with trusted publishing nowadays), or go ahead with #658 and have some docs like this alongside for the advanced usecase.

Personally I prefer leaving the setup to docs so it's completely transparent of how it's set up. It's worth for users to understand the .npmrc syntax etc.

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[Andarist]

I think a section like this is enough of a reason to go with #658 to, at least, ease common scenarios.

[bluwy]

These aren't common though, except yarn since it literally needs its own config and documentation, but then it's also a simple yarn config that you can create manually which isn't hard.