[UUF] Move HTTP example out of main overview page for STJ

[gewarren]

In response to UUF feedback: "Title is about dotnet serialization. The only example is with http methods, which is a very particular use case in dotnet. Not helpful if you want to serialize/ deserialize a file for instance. Or in a unit test."

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Internal previews

📄 File	🔗 Preview link
docs/core/whats-new/dotnet-5.md	What's new in .NET 5
docs/fundamentals/toc.yml	docs/fundamentals/toc
docs/standard/serialization/system-text-json/httpclient-extensions.md	Serialization extension methods on HttpClient
docs/standard/serialization/system-text-json/overview.md	JSON serialization and deserialization in .NET - overview

[BillWagner]

This LGTM @gewarren

Just address the anchor that moved and it's good to go.