[12.0 P1] Document new .NET 12 passkey features

### Description

Unblocks when `aspnetcore-12.0` exists on Learn.

### What needs documenting

Three features already merged into `dotnet/aspnetcore` `main`. All for .NET 12.

1. **Well-known passkey endpoints** (dotnet/aspnetcore#68066). `AddPasskeyEndpoints` and `MapWellKnownPasskeyEndpoints` serve a discovery document at `/.well-known/passkey-endpoints`. Marked `[Experimental("ASP0039")]`, which is a compile error and not a warning, so the suppression guidance has to be part of the docs. Mapping under a route group prefix logs an error rather than throwing.
2. **WebAuthn signals API** (dotnet/aspnetcore#68129). Three `SignInManager` methods plus matching `IPasskeyHandler` default interface members. The security points that need to survive into the prose: the browser treats the accepted-credentials list as authoritative, so an incomplete list permanently deletes working passkeys, and `signalUnknownCredential` also deletes permanently, which is why the method returns `null` on anything inconclusive.
3. **Reauthentication before adding a passkey** (dotnet/aspnetcore#68522). Template only, no new public API, but it moves `/Account/PasskeyCreationOptions` to `/Account/Manage/PasskeyCreationOptions` and adds `PasskeyOperation.Reauthenticate`.

### The content

The finished draft is the diff on #37565, 116 lines against `index.md`. It should still apply once the moniker exists, though every API name is worth rechecking against `PublicAPI` first.

### Page URL

https://learn.microsoft.com/en-us/aspnet/core/security/authentication/passkeys/?view=aspnetcore-10.0

### Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/security/authentication/passkeys/index.md

### Document ID

f44a13ca-c7a0-490a-9251-931ff16554f2

### Platform Id

c5da9dfd-32c6-e966-5a91-9426d957e69f

### Article author

@guardrex

### Metadata

* ID: f44a13ca-c7a0-490a-9251-931ff16554f2
* PlatformId: c5da9dfd-32c6-e966-5a91-9426d957e69f
* Service: **aspnet-core**
* Sub-service: **security**

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