docs migration - Going Passwordless with Passkeys

Source: documentation/server/guides/passkeys.md
Proposed disposition: Migrate
Proposed destination: server-guides/Sources/ServerGuides.docc/passkeys.md
Reviewer: @swiftlang/server-workgroup

Note: The article is tutorial-style and links to an external starter project on GitHub. Verify the starter project is still accessible and maintained before migrating.

Checklist

• Create a GitHub issue to track this migration
• Determine disposition (suggested: Migrate) and confirm destination
• Verify the external starter project (brokenhandsio/swift-webauthn-guide) is still accessible and up to date
• Review source for accuracy against current webauthn-swift library API
• Translate Jekyll markdown to DocC markdown format
  • Remove YAML front matter
  • Convert any Jekyll callouts to DocC-style callouts
  • Verify all WebAuthn registration and authentication flow steps
  • Update links to external resources
• Add article to ServerGuides.docc/Documentation.md under an "Authentication" or "Security" topic group
• Run swift package generate-documentation --analyze --warnings-as-errors in server-guides/
• Preview locally with swift package --disable-sandbox preview-documentation in server-guides/
• Submit PR to this repository for server workgroup review
• After merge and publication: add redirect_to: in the swift.org source file's front matter
• Submit redirect PR to swift-org-website

Start with documentation/server/guides/passkeys.md and compare it with server-guides/Sources/ServerGuides.docc/passkeys.md and the current webauthn-swift API. Verify brokenhandsio/swift-webauthn-guide is accessible and maintained, then migrate and link the article in ServerGuides.docc/Documentation.md. Run the two listed documentation commands; done means the guide builds, previews correctly, and is ready for workgroup review.