Document new warning NU3042

[dtivel]

Please do not merge yet. There are corresponding code changes that are still in review.

This change adds documentation for a new warning NU3042.

CC @JonDouglas, can you please review?

[nkolev92]

Where is NU3042 implemented?

It'll be great if all the warnings follow the prescribed methodology for warnings vs errors.
https://github.com/NuGet/NuGet.Client/blob/80c3c74ca5c6d67912a19833e8cc2b10f875dc95/src/NuGet.Core/NuGet.Common/Errors/NuGetLogCode.cs#LL21

I know a lot of the signing stuff doesn't, as lots of things are warnings in some cases, errors in others, but for true warnings we should consider it.

[dtivel]

@nkolev92, NU3042 is implemented here.

From day 0, signing codes took a different approach with respect to warning vs. error, since the same NU code can be a warning or error depending on signature verification settings. Instead, the codes are just assigned monotonically from NU3000.

[learn-build-service-prod]

Learn Build status updates of commit a913ec8:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/reference/Errors-and-Warnings.md	✅Succeeded	View
docs/reference/errors-and-warnings/NU3018.md	✅Succeeded	View
docs/reference/errors-and-warnings/NU3028.md	✅Succeeded	View
docs/reference/errors-and-warnings/NU3042.md	✅Succeeded	View
docs/TOC.md	✅Succeeded	View

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