doc: nrf: add wifi low-level API docs

Add docs from nrf_wifi.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/nrf/drivers/wifi.rst

@@ -8,3 +8,4 @@ Wi-Fi drivers
 
    wifi/nrf70_native
    wifi/nrf70_portable
+   wifi/low_level_api

doc/nrf/drivers/wifi/low_level_api.rst

@@ -0,0 +1,40 @@
+Low-Level API
+#############
+
+Common
+******
+
+The common API is used for both normal operation and Radio Test mode.
+
+| Header file: :file:`nrf_wifi/fw_if/umac_if/inc/fmac_structs_common.h`
+| Source file: :file:`nrf_wifi/fw_if/umac_if/src/fmac_api_common.c`
+
+
+.. doxygengroup:: nrf_wifi_api_common
+   :project: wifi
+
+Radio Test mode
+***************
+
+The Radio Test mode is used for testing the RF performance of the nRF70 Series device, mainly to characterize the RF.
+
+.. note::
+
+   Documentation for data structure definitions is disabled for this module.
+   It will be enabled in the future.
+
+| Source file: :file:`nrf_wifi/fw_if/umac_if/src/radio_test/fmac_api.c`
+
+.. doxygengroup:: nrf_wifi_api_radio_test
+   :project: wifi
+
+Wi-Fi mode
+**********
+
+The Wi-Fi mode is used for the normal operation of the nRF70 Series device in the :abbr:`STA (Station)` mode or other modes that will be supported in the future.
+
+| Header file: :file:`nrf_wifi/fw_if/umac_if/inc/default/fmac_api.h`
+| Source file: :file:`nrf_wifi/fw_if/umac_if/src/default/fmac_api.c`
+
+.. doxygengroup:: nrf_wifi_api
+   :project: wifi