WebAuthn conditional create is not documented

https://github.com/w3c/webappsec-credential-management/pull/224 added a `mediation` option to `CredentialsContainer.create()`, to enable websites to support [conditional create](https://developer.chrome.com/docs/identity/webauthn-conditional-create), AKA automatic passkey creation.

This should be documented on MDN.

In fact it would be good to rework https://developer.mozilla.org/en-US/docs/Web/API/Web_Authentication_API#discoverable_credentials_and_conditional_mediation, to make it clear that it's really about autofill (and to downplay the stuff about discoverable credentials), and move it into a separate guide page that also covered conditional create.