Advice about using netstandard2.0 or net5.0 (or newer) is unclear

Hello!

This page says that a library author should target `netstandard2.0` and ALSO `net5.0` if they need that specific functionality.
But [other documentation about netstandard](https://docs.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-1-0#when-to-target-net50-or-net60-vs-netstandard) says otherwise:

> If you don't need to support .NET Framework, you could go with .NET Standard 2.1 or .NET 5/6. We recommend you skip .NET Standard 2.1 and go straight to .NET 6.

I think this should be cleared up. And I think it would be good to only have 1 source of truth, perhaps one of these 2 pages need to have a reference to each other?

Also, should net5.0 be targeted? That's EOL...

I also think a table like this would be useful (example):

| goal | recommended versions (.netstandard, net6.0, etc..) | | | |
|------------------------------------------------------|----------------------------------------------------|---|---|---|
| create a new library that uses asp.net core features | RECOMMENDATIONS HERE | | | |
| create a library that supports .net framework | recommendations here | | | |
| create a library that supports the newest platforms | recommendations here | | | |

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* ID: 5b3638c9-ea3c-1c9e-2763-5c3e8e965663
* Version Independent ID: aff99d6b-9471-9277-160b-670ee8028b53
* Content: [Cross-platform targeting for .NET libraries](https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting)
* Content Source: [docs/standard/library-guidance/cross-platform-targeting.md](https://github.com/dotnet/docs/blob/main/docs/standard/library-guidance/cross-platform-targeting.md)
* Product: **dotnet-fundamentals**
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* Microsoft Alias: **jamesnk**