I think that they might be trying to say that you are asking how to drive a nail with a screwdriver. Yes, nails can be driven (yes, doppler shift in transmitted vs received wavelengths can be detected), but not with a screwdrived (or an NRF24l01, and an arduino...).
Study up on HB100 Doppler motion sensor (not really what you want, but will demonstrate doppler shift).
you will need to understand multi-stage amplification and wave shaping via op-amp.s and how to display the demonstrated frequency shift. The HB100 will detect movement by measuring doppler shift in xmit vs rcv RF signal via internal circuitry (and size/distance by maginitude of returned signal, but not very accurately or with much repeatability). The module, unfortunately, outputs a very low level sine signal. You amplify that output signal (in multiple stages to reduce noise artifacts), overdrive to clip the waveform, and provide an offset to make it a non-zero-crossing square wave, then present that signal to the arduino for something like PULSEIN...... since the output frequency range of the HB100 module sisomething less than 3IKHz, you could simplly connect the amplified output to a speaker and use the audio shift to demonstrate the doppler effect.
However, as has been stated multiple times, give up on the idea of using the NRF24L01.
By the way, I have outlined a possible solution to whatever your project really is, and I have about one twentyith the technical expertise of the guys who have been telling you the Nordic radio will not work for this.