docs: update renamed deviceauthn fields references

[pcaillaudm]

This PR updates the device authentication (deviceauthn) documentation to match the renamed settings-flow API fields.

Changes:

• Key registration: the add payload field is renamed to deviceauthn_register, and the SDK
  model UpdateSettingsFlowWithDeviceAuthnMethodAdd is now
  UpdateSettingsFlowWithDeviceAuthnMethodRegister (method.add → method.deviceauthnRegister).
• Key removal: the delete object with a nested client_key_id is replaced by a flat
  deviceauthn_remove string field, removing the need for the
  UpdateSettingsFlowWithDeviceAuthnMethodDelete wrapper in the SDKs.
• The Android and iOS registration examples now include the required version: 1 field,
  aligning them with the payload reference in the overview page.

All JSON payloads, Kotlin/Swift SDK snippets, and prose references to the old field names
were updated accordingly.

[unatasha8]

Isn't renaming fields in the API something that can break customer code? Have we added a note or given some heads up beyond updating the docs?