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 https://github.com/dotnet/AspNetCore.Docs/issues/30215

[layomia]

@jkotas in https://github.com/dotnet/runtime/issues/79527#issuecomment-1706986512.

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.

[ericstj]

@IEvangelist @gewarren is it possible to prioritize this for .NET 9? Seems we still don't have good conceptual docs for the Configuration source generator.

[IEvangelist]

Yes, I'll commit to making that happen for .NET 9 for sure. I've been buried with .NET Aspire docs work lately, and haven't had lots of time to work on .NET docs for a while now.