doxygen: add groups for all modules

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/nrf24l01/nrf24l01.h

@@ -150,11 +150,17 @@ typedef enum {
     NRF_18DBM   = 3,
 } power_t;
 
+/**
+ * @brief nrf24l01 transceiver library
+ * @defgroup nrf24l01 libupm-nrf24l01
+ */
+
 /**
  * @brief C++ API for NRF24l01 transceiver module
  *
  * This file defines the NRF24l01 C++ interface for libnrf24l01
  *
+ * @ingroup nrf24l01
  * @snippet nrf_receiver.cxx Interesting
  * @snippet nrf_transmitter.cxx Interesting
  */