Add scenarios to the System.Text.Json how-to doc

[tdykstra]

Related to #12207

From comments on the #14208 PR

• Add an example about converting enums to their label names (rather than numbers) using JsonStringEnumConverter. https://docs.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonstringenumconverter?view=netcore-3.0
• Add an example where a default value is not overridden by null on deserialize when IgnoreNullValues is set to true.
• Add a section on JsonDocument/JsonElement between the serializer/deserializer and the reader/writer.

From the doc planning meeting

• Write to and read from a file, synchronous and async
• Write to and read from a Stream, synchronous and async
• Change the encoding, for example, to allow a Unicode language character set.

The list of scenarios to add has been edited to include only the ones that were added by PR #15113. New issue #15805 was created to track the outstanding scenarios.

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Document Details

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• ID: 588ae32f-3429-143f-824c-5ab2d046d24a
• Version Independent ID: 002f9f2a-48d1-0671-a079-b99b9f6bab58
• Content: How to serialize JSON - .NET
• Content Source: docs/standard/serialization/system-text-json-how-to.md
• Product: dotnet
• GitHub Login: @tdykstra
• Microsoft Alias: tdykstra