Updates to Migrate from Newtonsoft.Json doc

[tdykstra]

From comments in the PR that created the doc:

• Add stream sync to async examples
• More quoted number examples5.0 feature
• Periodically call FlushAsync
• For Merge sample, change issue link to code example
• No customization like FloatFormatHandling in the Utf8JsonWriter
• Add sub-tocs for h3 sections See Create table of System.Text.Json - Newtonsoft.Json equivalents #16309

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Document Details

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• ID: 6b74d77c-33e1-d44c-18c6-bd0b612f74de
• Version Independent ID: 9c6a7135-eb75-21a9-1d27-0880459523ca
• Content: Migrate from Newtonsoft.Json to System.Text.Json - .NET
• Content Source: docs/standard/serialization/system-text-json-migrate-from-newtonsoft-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.