More System.Text.Json updates for .NET 9

[gewarren]

Contributes to #42592.

• Read multiple JSON documents using Utf8JsonReader.
• Indentation options.
• DeepEquals for JsonElement and JsonNode.
• Update code snippets to use Web options singleton.
• Add links from What's new.

---

Internal previews

📄 File	🔗 Preview link
docs/core/whats-new/dotnet-9/libraries.md	docs/core/whats-new/dotnet-9/libraries
docs/standard/serialization/system-text-json/extract-schema.md	docs/standard/serialization/system-text-json/extract-schema
docs/standard/serialization/system-text-json/handle-overflow.md	docs/standard/serialization/system-text-json/handle-overflow
docs/standard/serialization/system-text-json/how-to.md	docs/standard/serialization/system-text-json/how-to
docs/standard/serialization/system-text-json/migrate-from-newtonsoft.md	docs/standard/serialization/system-text-json/migrate-from-newtonsoft
docs/standard/serialization/system-text-json/use-dom.md	docs/standard/serialization/system-text-json/use-dom
docs/standard/serialization/system-text-json/use-utf8jsonreader.md	docs/standard/serialization/system-text-json/use-utf8jsonreader
docs/standard/serialization/system-text-json/visual-basic-support.md	docs/standard/serialization/system-text-json/visual-basic-support

[BillWagner]

This LGTM @gewarren

Once you address the open comments, let's