Add link to Migrate-from-Newtonsoft doc (#16678)

* Add link to Migrate-from-Newtonsoft doc

* Update docs/core/whats-new/dotnet-core-3-0.md

Co-Authored-By: Ahson Khan <[email protected]>

* Update docs/core/whats-new/dotnet-core-3-0.md

Co-Authored-By: Ahson Khan <[email protected]>

Co-authored-by: Ahson Khan <[email protected]>

Author: tdykstra

docs/core/whats-new/dotnet-core-3-0.md

@@ -492,9 +492,13 @@ System.Console.WriteLine($"RuntimeInformation.FrameworkDescription: {System.Runt
 
 ### Fast built-in JSON support
 
-.NET users have largely relied on [**Json.NET**](https://www.newtonsoft.com/json) and other popular JSON libraries, which continue to be good choices. **Json.NET** uses .NET strings as its base datatype, which is UTF-16 under the hood.
+.NET users have largely relied on [Newtonsoft.Json](https://www.newtonsoft.com/json) and other popular JSON libraries, which continue to be good choices. `Newtonsoft.Json` uses .NET strings as its base datatype, which is UTF-16 under the hood.
 
-The new built-in JSON support is high-performance, low allocation, and based on `Span<byte>`. For more information about the <xref:System.Text.Json> namespace and types, see [JSON serialization in .NET - overview](../../standard/serialization/system-text-json-overview.md). For tutorials on common JSON serialization scenarios, see [How to serialize and deserialize JSON in .NET](../../standard/serialization/system-text-json-how-to.md).
+The new built-in JSON support is high-performance, low allocation, and works with UTF-8 encoded JSON text. For more information about the <xref:System.Text.Json> namespace and types, see the following articles:
+
+* [JSON serialization in .NET - overview](../../standard/serialization/system-text-json-overview.md)
+* [How to serialize and deserialize JSON in .NET](../../standard/serialization/system-text-json-how-to.md).
+* [How to migrate from Newtonsoft.Json to System.Text.Json](../../standard/serialization/system-text-json-migrate-from-newtonsoft-how-to.md)
 
 ### HTTP/2 support