Add JSON schema exporter article

[gewarren]

New article preview

• Contributes to Create/Update conceptual docs for new System.Text.Json features in .NET 9 #42592.
• Also moves Copilot section to end of "Migrate from Newtonsoft.Json" article based on UUF feedback.

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Internal previews

📄 File	🔗 Preview link
docs/standard/serialization/system-text-json/character-casing.md	docs/standard/serialization/system-text-json/character-casing
docs/standard/serialization/system-text-json/character-encoding.md	docs/standard/serialization/system-text-json/character-encoding
docs/standard/serialization/system-text-json/configure-options.md	docs/standard/serialization/system-text-json/configure-options
docs/standard/serialization/system-text-json/handle-overflow.md	docs/standard/serialization/system-text-json/handle-overflow
docs/standard/serialization/system-text-json/json-schema-exporter.md	docs/standard/serialization/system-text-json/json-schema-exporter
docs/standard/serialization/system-text-json/migrate-from-newtonsoft.md	docs/standard/serialization/system-text-json/migrate-from-newtonsoft

[eiriktsarpalis]

Thanks!

[BillWagner]

This LGTM @gewarren

Once you address the remaining comments, let's