First-pass porting of System.Text.Json API docs.

[eiriktsarpalis]

NB more passes will be necessary to account for RC1 APIs that haven't been ingested yet + any missing doc elements that will be filled out manually.

Contributes to dotnet/runtime#88577

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json
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Issue Details

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NB more passes will be necessary to account for RC1 APIs that haven't been ingested yet + any missing doc elements that will be filled out manually.

Contributes to #88577

Author:	eiriktsarpalis
Assignees:	eiriktsarpalis
Labels:	area-System.Text.Json
Milestone:	-

[carlossanlop]

Left some comments. @gewarren I'll apply them myself since @eiriktsarpalis is currently OOF.

[learn-build-service-prod]

Learn Build status updates of commit 0ee79e7:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Text.Json.Nodes/JsonArray.xml	✅Succeeded	View
xml/System.Text.Json.Nodes/JsonNode.xml	✅Succeeded	View
xml/System.Text.Json.Serialization.Metadata/JsonMetadataServices.xml	✅Succeeded	View
xml/System.Text.Json.Serialization.Metadata/JsonPropertyInfo.xml	✅Succeeded	View
xml/System.Text.Json.Serialization.Metadata/JsonTypeInfo.xml	✅Succeeded	View
xml/System.Text.Json.Serialization.Metadata/JsonTypeInfoResolver.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonConverter.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonKnownNamingPolicy.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonNumberEnumConverter`1.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonObjectCreationHandling.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonObjectCreationHandlingAttribute.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonSourceGenerationOptionsAttribute.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonStringEnumConverter`1.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonUnmappedMemberHandling.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonUnmappedMemberHandlingAttribute.xml	✅Succeeded	View
xml/System.Text.Json/JsonEncodedText.xml	✅Succeeded	View
xml/System.Text.Json/JsonNamingPolicy.xml	✅Succeeded	View
xml/System.Text.Json/JsonSerializer.xml	✅Succeeded	View
xml/System.Text.Json/JsonSerializerOptions.xml	✅Succeeded	View
xml/System.Text.Json/Utf8JsonWriter.xml	✅Succeeded	View

For more details, please refer to the build report.

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