Initial .NET 6 Preview updates for .System.Text.Json

[tdykstra]

• Adds .NET 6 Preview zone pivot on the right so .NET 5 remains the default for now
• Fixes Mention JsonNode as a .NET 6 alternative to JsonElement #24619
• Fixes Document ReferenceHandler.IgnoreCycles #24633
• Adds placeholders linking to the Preview 4 blog for Preview 4 features not yet documented.

Preview Links:

File Type	File Name	Published Url
Content	docs/standard/serialization/system-text-json-character-casing.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-character-casing?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-character-encoding.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-character-encoding?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-configure-options.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-configure-options?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-converters-how-to.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-customize-properties.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-customize-properties?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-handle-overflow.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-handle-overflow?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-how-to.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-how-to?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-ignore-properties.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-ignore-properties?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-immutability.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-immutability?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-invalid-json.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-invalid-json?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-overview.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-overview?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-polymorphism.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-polymorphism?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-preserve-references.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-preserve-references?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-supported-collection-types.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-supported-collection-types?branch=pr-en-us-24514
Content	docs/standard/serialization/system-text-json-use-dom-utf8jsonreader-utf8jsonwriter.md	https://review.docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-use-dom-utf8jsonreader-utf8jsonwriter?branch=pr-en-us-24514

[luisquintanilla]

LGTM

[layomia]

LGTM