Documentation for CodePagesEncodingProvider is out of date

This page has several issues:

1. It claims `CodePagesEncodingProvider` is not available for .Net Core 2.1, which is not true. (In fact, it's a NuGet package, so AFAIK it's not part of any .Net TFM, but works in any .Net Standard 1.3+ TFM, if installed. But I think that's a larger issue that's out of scope here.)
2. It claims it's only relevant for .Net Native. That's not true, `CodePagesEncodingProvider` is also required for some code pages on regular .Net Core.
3. It says that you should reference the System.Text.Encoding.CodePages.dll assembly. That might work on UWP (not sure), but a more general approach is to reference the System.Text.Encoding.CodePages package from NuGet.

This issue was inspired by https://github.com/dotnet/docs/issues/6931.

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* ID: a5c26a9c-9378-f153-0e10-2d893d8ea178
* Version Independent ID: 5a122d68-cd0b-abe8-fd6c-258d44c816a2
* Content: [CodePagesEncodingProvider Class (System.Text)](https://docs.microsoft.com/en-us/dotnet/api/system.text.codepagesencodingprovider?view=netcore-2.0&viewFallbackFrom=netcore-2.1)
* Content Source: [xml/System.Text/CodePagesEncodingProvider.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Text/CodePagesEncodingProvider.xml)
* Product: **dotnet-api**
* GitHub Login: @rpetrusha
* Microsoft Alias: **ronpet**