boards: arm: sensortile_box_pro: add bluenrg-lp chip description

Extend sensortile_box_pro document adding BlueNRG-LP chip description.

Signed-off-by: Armando Visconti <armando.visconti@st.com>

boards/st/sensortile_box_pro/doc/index.rst

@@ -41,6 +41,9 @@ sensors on board)
 Hardware
 ********
 
+The following is a summary of the main board features. More info can be find on `UM3133`_
+and the `schematic`_.
+
 The STM32U585xx devices are an ultra-low-power microcontrollers family (STM32U5
 Series) based on the high-performance Arm|reg| Cortex|reg|-M33 32-bit RISC core.
 They operate at a frequency of up to 160 MHz.
@@ -212,6 +215,15 @@ The final application may use it to declare SensorTile.box PRO device as belongi
 certain standard or vendor class, e.g. a CDC, a mass storage or a composite device with both
 functions.
 
+BlueNRG-LP chip
+===============
+
+The board is equipped with an STMicroelectronics `BlueNRG-LP`_ chip. Before using the board Bluetooth
+functionality, a firmware upgrade of the chip may be necessary, in order to run a firmware which is
+compatible with the Zephyr BT stack (see `stsw-mkbox-bleco`_ for more information).
+The upgrade may be easily performed following the procedure explained in
+:zephyr:code-sample:`sensortile_box_pro_sample_ble_fwupg` sample.
+
 Console
 =======
 
@@ -333,12 +345,21 @@ References
 .. _SensorTile.box PRO website:
    https://www.st.com/en/evaluation-tools/steval-mkboxpro.html
 
+.. _UM3133:
+   https://www.st.com/resource/en/user_manual/um3133-getting-started-with-sensortilebox-pro-multisensors-and-wireless-connectivity-development-kit-for-any-intelligent-iot-node-stmicroelectronics.pdf
+
+.. _schematic:
+   https://www.st.com/resource/en/schematic_pack/steval-mkboxpro-schematic.pdf
+
 .. _STM32U585 on www.st.com:
    https://www.st.com/en/microcontrollers-microprocessors/stm32u575-585.html
 
 .. _STM32U585 reference manual:
    https://www.st.com/resource/en/reference_manual/rm0456-stm32u575585-armbased-32bit-mcus-stmicroelectronics.pdf
 
+.. _BlueNRG-LP:
+   https://www.st.com/en/wireless-connectivity/bluenrg-lp.html
+
 .. _lsm6dsv16x datasheet:
    https://www.st.com/en/mems-and-sensors/lsm6dsv16x.html
 
@@ -362,3 +383,6 @@ References
 
 .. _DFU-UTIL website:
    http://dfu-util.sourceforge.net/
+
+.. _stsw-mkbox-bleco:
+   https://www.st.com/en/embedded-software/stsw-mkbox-bleco.html