Some guidance to enable NRT for library that is compatible with .NET and .NET Framework

I search some guidance to enable NRT (nullable reference type) in my library, but I found nothing in [Library guidance](https://learn.microsoft.com/en-us/dotnet/standard/library-guidance/).

I found this blog post [Embracing nullable reference types](https://devblogs.microsoft.com/dotnet/embracing-nullable-reference-types) that have guidance, but it is from 2019. This is old and this guidance can be obsolete.

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I would love to see a guidance in the doc to explain how enable NRT in a library that need to be compatible with .NET Framework and .NET.

If the below blog post is still relevant, I can help and make a PR.

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* ID: b466e1d8-f9fc-6a4b-50a3-a101e5ea9907
* Version Independent ID: ccf98d0b-d65c-522a-a944-e58d60adbee8
* Content: [Get started creating high-quality .NET libraries - .NET](https://learn.microsoft.com/en-us/dotnet/standard/library-guidance/get-started)
* Content Source: [docs/standard/library-guidance/get-started.md](https://github.com/dotnet/docs/blob/main/docs/standard/library-guidance/get-started.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @JamesNK
* Microsoft Alias: **jamesnk**