[Pre6] Passkeys coverage

[guardrex]

@mikekistler @MackinnonBuck ... You can place the article content into an issue comment, and I'll see to it that it get placed by Thursday.

References ...

• Update "What's new in ASP.NET Core in .NET 10" for .NET 10 Preview 6 #35602 (comment)
• [10.0.1xx-preview6] Add passkeys to ASP.NET Core Identity dotnet#1237

Coverage options:

• Minimum coverage: What's New coverage with app config guidance, mostly based on what was placed for the .NET Core release notes. A reference article would be added for a later preview after the feature bakes a bit more.
• Normal reference coverage: Security & Identity> Authentication > ASP.NET Core Identity, if the feature is bound to Identity. Otherwise, the article probably goes out in the Security & Identity > Authentication node.
• If Blazor/Razor components have special requirements that require separate coverage, an article would be added to Blazor's Security & Identity node. It doesn't seem like this will be required for this feature.