STJ: Populate properties

[gewarren]

Contributes to #37101

• Splits out the how-to article into separate serialization/deserialization articles
• Reorganizes the STJ table of contents to put topics specific to serialization or deserialization under those nodes
• Adds new article on populating properties
• Updates the immutability article to include read-only properties
• Updates the migrate from NewtonSoft article

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Internal previews

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📄 File	🔗 Preview link
docs/core/whats-new/dotnet-5.md	What's new in .NET 5
docs/csharp/programming-guide/strings/index.md	Strings and string literals
docs/standard/serialization/index.md	Serialization in .NET
docs/standard/serialization/system-text-json/customize-properties.md	How to customize property names and values with System.Text.Json
docs/standard/serialization/system-text-json/deserialization.md	How to read JSON as .NET objects (deserialize)
docs/standard/serialization/system-text-json/fields.md	docs/standard/serialization/system-text-json/fields
docs/standard/serialization/system-text-json/how-to.md	How to write .NET objects as JSON (serialize)
docs/standard/serialization/system-text-json/ignore-properties.md	How to ignore properties with System.Text.Json
docs/standard/serialization/system-text-json/immutability.md	Use immutable types and properties
docs/standard/serialization/system-text-json/migrate-from-newtonsoft.md	Migrate from Newtonsoft.Json to System.Text.Json
docs/standard/serialization/system-text-json/overview.md	docs/standard/serialization/system-text-json/overview
docs/standard/serialization/system-text-json/polymorphism.md	How to serialize properties of derived classes with System.Text.Json
docs/standard/serialization/system-text-json/populate-properties.md	Populate initialized properties
docs/standard/serialization/system-text-json/supported-collection-types.md	Supported collection types in System.Text.Json
docs/standard/serialization/system-text-json/use-utf8jsonreader.md	docs/standard/serialization/system-text-json/use-utf8jsonreader

[eiriktsarpalis]

LGTM pending feedback.

[eiriktsarpalis]

Tagging @krwq for review since he worked on the feature.

[BillWagner]

This LGTM @gewarren

I had one clarifying question, but I'm not certain it's an implementation detail that we shouldn't specify.