Add instructions how to add device to nRF Cloud

[coderbyheart]

I think it would be a good addition to the yellow box to specifically mention the steps on how to get it onto nRF Cloud.

Using nRF Connect for Desktop

For the Thingy:91 X we have changed the onboarding process, it now uses the attestation token method.
The easiest way is to use nRF Connect for Desktop, which has a QuickStart app, that helps with onboarding your device to your nRF Cloud account.
I hope this helps you trying out nRF Cloud.

Use multi-services sample

use the multiservice sample with west build -p -b thingy91x/nrf9151/ns -- -DEXTRA_CONF_FILE="overlay-http_nrf_provisioning.conf" and go through the relevant steps at Managing claimed devices using the nRF Cloud portal