Adds nuget.config behaviour section

[redging-very-well]

Adds a new NuGet.Config section which includes the otherwise undocumented behaviour of dotnet restore with the globalPackagesFolder setting.

This is an alternative to users who rely upon the previous repositoryPath behaviour.

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[dnfclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
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[guardrex]

@Redgwell Thank you for pointing this out! The dotnet restore command doc probably isn't the best spot for this, as the command doc should probably only cover its direct usage. A link to it here would be nice once it finds a home. I've moved this to an issue for further discussion. If there's good spot for it, you can submit the PR on it, but let's see if we can find where this goes. Moved to #1730

[redging-very-well]

Thanks @guardrex - I wasn't 100% sure that this was an appropriate place for this, so thanks for moving it over. I'll track the issue you've raised.