Make the polymorphic converter example multi-level

[tdykstra]

See this comment thread in the PR that created the Migrate from Newtonsoft.Json doc.

---

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 5de37d79-751f-e372-e866-efde28f4908c
• Version Independent ID: 678c4f23-764a-7ccd-0d6d-159a30957477
• Content: How to write custom converters for JSON serialization - .NET
• Content Source: docs/standard/serialization/system-text-json-converters-how-to.md
• Product: dotnet
• GitHub Login: @tdykstra
• Microsoft Alias: tdykstra

[dotnet-bot]

This issue has been closed as part of the issue backlog grooming process outlined in #22351.

That automated process may have closed some issues that should be addressed. If you think this is one of them, reopen it with a comment explaining why. Tag the @dotnet/docs team for visibility.