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

[tdykstra]

Related to #12207

• Add a sample of using ArrayBufferWriter<byte> as the output-sink since that helps avoid the extra encoding/allocation when trying to write UTF-8 text to a stream (say FileStream). https://github.com/dotnet/docs/pull/14208/files#r324126902
• Add JsonWriter samples https://github.com/dotnet/docs/pull/14208/files#r324127488
  • Write really large JSON file - say 3 GB.
  • Write asynchronously with periodic flushing.
  • [advanced/lower priority] Writing to a System.IO.Pipeline.PipeWriter
  • For more info, see https://gist.github.com/ahsonkhan/f6f30656717548212693e5eaa49cece5
• Add JsonReader samples https://github.com/dotnet/docs/pull/14208/files#r324129542
  • Reading really large JSON file - say 3 GB.
  • Reading asynchronously (using the state for continuation).
  • [advanced/lower priority] Reading from a System.IO.Pipeline.PipeReader
  • The custom options (comment handling - skipping/allow, trailing commas, max depth)
  • For more info, see
    • https://github.com/dotnet/apireviews/blob/master/2018/System.Text.Json/Additional.Notes.md
    • https://gist.github.com/ahsonkhan/f6f30656717548212693e5eaa49cece5
• Add a sample that reads from a Stream or file with multiple objects. See comment in PR 16225
• To build a JsonDocument from scratch, write JSON text by using the Utf8JsonWriter and parse the output from that to make a new JsonDocument.

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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 and deserialize JSON using C# - .NET
• Content Source: docs/standard/serialization/system-text-json-how-to.md
• Product: dotnet
• GitHub Login: @tdykstra
• Microsoft Alias: tdykstra

[tdykstra]

Closing, will replace with individual issues to add samples as we get requests for them.