Create/Update conceptual docs for new System.Text.Json features in .NET 8

[eiriktsarpalis]

Creating an issue to track creating new STJ conceptual docs in .NET 8, here's a list of key new features:

• Unmapped member handling.
• Update the source-generation-modes article to include sections on TypeInfoResolverChain, JsonSerializerOptions.IsReflectionEnabledByDefault, JsonStringEnumConverter<TEnum> and the JsonSourceGenerationOptionsAttribute extensions. Document the minimum language version (which has now been explicitly pegged to C# 9, cf. Add unsupported language detection to the STJ source generator runtime#87950).
• Populating read-only properties in JSON. This feature carries a lot of nuance warranting documentation, @krwq has done a great job capturing the nitty-gritty details in the original feature proposal.
  • This should additionally capture the problems of this feature with [parameterized constructors]
• Update the customize-properties document to include mention for snake_case and kebab-case naming policies. This is also tracked by Document behavior of the newly added System.Text.Json snake case and kebab case policies #36652.
• Update the migrate-from-newtonsoft guide to mark the following as supported:
  • MissingMemberHandling"
  • "Snake-case property names"
  • "Add to collections without setters" features
• Document recent changes to JsonIncludeAttribute/JsonConstructorAttribute #36128
• Document missing diagnostics (e.g. SYSLIB1034 and SYSLIB1039 and SYSLIB1220-SYSLIB1224) in https://learn.microsoft.com/en-us/dotnet/fundamentals/syslib-diagnostics/source-generator-overview (cf. https://github.com/dotnet/runtime/blob/main/docs/project/list-of-diagnostics.md)

I've posted an article summarizing all new STJ features which can be used to feed into the content proposed above as well as other new conceptual docs where appropriate.

cc @gewarren @IEvangelist

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Associated WorkItem - 164994