docs: add new section about GPS mode UI (#5438)

* docs: add new section about GPS

* docs: add note about availablitiy

docs/gateway-configuration/cellular-configuration.md

@@ -64,9 +64,24 @@ The **Cellular** tab contains the following configuration parameters:
 
 - **LCP Echo Failure**: sets the _lcp-echo-failure_ option of the PPP daemon. If set to a positive number, the modem presumes the peer to be dead if a specified number of LCP echo-requests are sent without receiving a valid LCP echo-reply. To disable this option, set it to zero.
 
-The **GPS** tab allows the user to enable or disable the GPS module provided by the cellular modem. The property is:
+### GPS
 
-- **Enable GPS**: enables GPS with the following conditions:
-    - One modem port will be dedicated to NMEA data stream.
-    - This port may not be used to send AT commands to the modem.
-    - The _PositionService_ should be enabled. Serial settings of the _PositionService_ should not be changed; it will be redirected to the modem GPS port automatically.
+![](./images/IMG-14-10-2024-11-27-34.png)
+
+The **GPS** tab allows the user to enable or disable the GPS module provided by the cellular modem. The available properties are:
+
+- **Enable GPS**: enables GPS module for the selected modem.
+- **GPS Mode**: specifies the GPS mode.
+    - `UNMANAGED`: the GPS device of the modem will be setup but not directly managed, therefore freeing the serial port for other services to use. This can be used in order to perform the setup of the GPS and then have another service (like `gpsd`) parse the NMEA strings in order to extract the position informations.
+    - `MANAGED_GPS`: the GPS device of the modem will be setup and directly managed (typically by ModemManager) therefore the serial port won't be available for other services to use.
+
+!!! note "GPS modes availability"
+    GPS modes available for the modem are dependent on the modem model, modem firmware version and _ModemManager_ version installed on the system. Some modes may not be selectable if the modem does not support them.
+
+Therefore, to use the GPS module provided by the cellular modem with Kura's _PositionService_, the following considerations should be taken into account:
+
+- The _PositionService_ should be enabled. Serial settings of the _PositionService_ should not be changed; it will be redirected to the modem GPS port automatically.
+- To use the `gpsd` and `serial` _PositionService_ providers with the GPS module provided by the cellular modem, the GPS mode should be set to `UNMANAGED`.
+- To use the `modemmanager` _PositionService_ provider with the GPS module provided by the cellular modem, the GPS mode should be set to `MANAGED_GPS`.
+
+Refer to the [Position Service](../core-services/position-service.md) section for more information.