feat: add Passkey APIs to Authentication API reference

[tomauth0]

Description

Adds reference documentation for the new Passkey APIs (POST /passkey/challenge
and POST /passkey/register) under the Authentication API, in support of the
Embedded Capabilities GA relaunch (Passkey APIs going GA — covers native iOS,
native Android, and web).

What's in this PR:

• main/docs/api/authentication/passkey/challenge.mdx — documents
  POST /passkey/challenge. Covers the request body (client_id,
  client_secret, realm, organization), prerequisites (custom domain,
  first-party + OIDC-conformant client, passkey grant enabled), and the
  WebAuthn PublicKeyCredentialRequestOptions returned in
  authn_params_public_key alongside the auth_session value used in the
  subsequent token exchange.
• main/docs/api/authentication/passkey/register.mdx — documents
  POST /passkey/register. Covers user_profile signup attributes and
  validation rules, user_metadata constraints (max 10 fields, reserved
  field-name blocklist), and the WebAuthn PublicKeyCredentialCreationOptions
  returned (rp, user, pubKeyCredParams, authenticatorSelection).
• main/config/navigation/generated/authentication.en.json — adds a new
  "Passkeys" group between "Passwordless" and "Device Authorization" that
  links to both new pages.

Testing

• Run mint dev from main/ and confirm:
  • The "Passkeys" group appears in the Authentication API navigation between
    "Passwordless" and "Device Authorization".
  • Both /passkey/challenge and /passkey/register pages render with
    parameter, response field, and status code tables.
  • Internal links to Passkeys, Configure Passkey Policy, and Custom Domains
    resolve.

Checklist

• I've read and followed CONTRIBUTING.md.
• I've tested the site build for this change locally.
• I've made appropriate docs updates for any code or config changes.
• I've coordinated with the Product Docs and/or Docs Management team about non-trivial changes.

[avanscoy]

Suggested change

	User's email address. Validated against the JSON Schema email format.
	User's email address. Validated against the `JSON` schema email format.

[avanscoy]

Suggested change

	User's username. Validated against the connection's username policy (length and allowed characters).
	User's username. Validates against the connection's username policy (length and allowed characters).

[hazel-nut]

lgtm, just some alignment with the existing auth api docs :)