Extend config extensions doc with binding source generator info

[layomia]

• https://devblogs.microsoft.com/dotnet/announcing-dotnet-8-preview-6/#configuration-binding-source-generator-improvements
• https://learn.microsoft.com/en-us/dotnet/core/extensions/configuration
• https://github.com/dotnet/docs/blob/b8f805b78dc90ca6339d18d0d404a3fff9c01453/docs/core/extensions/configuration.md
• ASP.NET Extend configuration doc with binding source generator info AspNetCore.Docs#30215

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Associated WorkItem - 317207

[layomia]

@jkotas in dotnet/runtime#79527 (comment).

Also, we should have this source generator documented at https://learn.microsoft.com/en-us/dotnet/core/extensions/configuration. Once the documentation is available, please update this link to point to it.