docs(instance): update recommended image usage

[Codelax]

Currently we can use an image label when creating an instance, we should use the marketplace datasource to resolve it.
From:

resource scaleway_instance_server server {
  image = "ubuntu_jammy"
}

To:

data scaleway_marketplace_image ubuntu {
  label = "ubuntu_jammy"
  commercial_type = "DEV1-S"
}

resource scaleway_instance_server server {
  image = data.scaleway_marketplace_image.ubuntu
}