Update passwordless-lib URL and add its demo

[Regenhardt]

Not entirely sure how to add the demo, there's an actual demo site too (https://fido2-net-lib.passwordless.dev/) where the linked demo is hosted, but it doesn't fit the concept of linking the Github repository.

[CLAassistant]

All committers have signed the CLA.

[MasterKale]

This looks good to me, "Previewing WebAuthn" is intended to be a place to capture experiences like the one you're adding in here.

[MasterKale]

@Regenhardt Your demo is now live on https://webauthn.io :)