Add Kotlin WebAuthn library to libraries page

[szijpeter]

This adds webauthn-kotlin-multiplatform under the "Updated for passkeys" section as a Kotlin option.

Why it seems like a fit:

• open-source WebAuthn/passkey library
• Kotlin Multiplatform with published artifacts
• server-side WebAuthn support plus Android/iOS client support
• actively maintained and recently released publicly

Repo: https://github.com/szijpeter/webauthn-kotlin-multiplatform

[timcappalli]

This is a very new GitHub repo/project, so I'm hesitant to merge this right away, as I'm unable to test it right now.

There's also two GitHub accounts appearing to represent the same person in this PR.

[szijpeter]

Thanks for the feedback, and apologies as well for the wrong signature on the original commit — that is now corrected.

As you pointed out, the repo is still in its early stages, so please feel free to keep this on hold or reject the PR at your convenience.