Link to NuGet security best practices

David Britch pointed out that the topic on security best practices related to managing packages could be better surfaced. I agree. Perhaps linking directly to the actionable steps starting with this section:

[Best practices for a secure software supply chain: Knowing what is in your environment](https://docs.microsoft.com/nuget/concepts/security-best-practices#knowing-what-is-in-your-environment)

Maybe link to it from our security section, and/or when we first introduce the concept of packages. (Discussion welcome as always.)