sample/dfu/fw_loader/usb_mcumgr: add sample doc

Doc added (pattern for ble_mcumgr repalyed).

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

Author: nvlsianpu

samples/dfu/fw_loader/usb_mcumgr/README.rst

@@ -0,0 +1,55 @@
+.. _fw_loader_usb_mcumgr:
+
+Minimal USB virtual serial port SMP firmware loader
+###################################################
+
+.. contents::
+   :local:
+   :depth: 2
+
+This sample provides the minimal and recommended configuration for the Firmware Loader application to run on the :zephyr:board:`nrf54lm20dk`.
+It uses code from Zephyr's :zephyr:code-sample:`smp-svr` sample to enable the USB serial Simple Management Protocol (SMP) server functionality.
+
+This sample is not intended to function as a standalone sample.
+Instead, it serves as a starting point for developing a custom Firmware Loader application that works with the MCUboot bootloader.
+
+.. _fw_loader_usb_minimal_overview:
+
+Requirements
+************
+
+The sample supports the following development kit:
+
+.. table-from-sample-yaml::
+
+.. _fw_loader_usb_minimal_build_run:
+
+Overview
+********
+
+The application uses the SMP over Bluetooth LE to perform firmware updates and manage device operations.
+It is optimized for minimal memory usage and contains only the essential functionalities required for firmware loading.
+
+The firmware loader over USB CDC ACM virtual serial port accepts SMP commands for:
+
+* Image upload and management
+* Device information queries
+* Bootloader information
+
+.. _fw_loader_usb_minimal_reqs:
+
+Building and running
+********************
+
+This sample is not intended for standalone building.
+It is automatically included and built when the following Kconfig options are enabled in a project configuration:
+
+* :kconfig:option:`SB_CONFIG_BOOTLOADER_MCUBOOT`
+* :kconfig:option:`SB_CONFIG_MCUBOOT_MODE_FIRMWARE_UPDATER`
+* :kconfig:option:`SB_CONFIG_FIRMWARE_LOADER_IMAGE_USB_MCUMGR`
+
+
+Testing
+=======
+
+This sample is not intended for standalone testing.

samples/dfu/single_slot/README.rst

@@ -34,8 +34,8 @@ Building and running
 
 .. |sample path| replace:: :file:`samples/dfu/single_slot`
 
-Sample by defult is building with :ref:`fw_loader_ble_mcumgr` firmware loader image.
-To build with :ref:`fw_loader_usb_mcumgr` firmware loader image, use the following provide FILE_SUFFIX=usb to the build command.
+By default, the sample builds with the :ref:`fw_loader_ble_mcumgr` firmware loader image.
+To build with the :ref:`fw_loader_usb_mcumgr` firmware loader image, append ``FILE_SUFFIX=usb`` to the build command.
 
 .. include:: /includes/build_and_run.txt
 
@@ -52,13 +52,22 @@ After programming the sample to your development kit, perform the following step
 
 #. Build a second version of the sample.
 #. Enter the firmware loader mode by holding the **Button 0** on your development kit while you reset the device.
-#. For Bluetooth firmware loader: Open the `nRF Connect Device Manager`_ mobile app to perform DFU over Bluetooth LE.
-   The firmware loader advertises itself as **FW loader** and accepts MCUmgr image upload.
-   #. Send the generated update package for the second version of the sample.
-      See :ref:`ug_nrf54l_developing_ble_fota_steps_testing` for details on how to use the mobile app to perform the DFU.
-#. For USB CDC ACM serial firmware loader: Use `nRF Util mcu-manager serial` to perform DFU over serial port.
-   #. Send the generated update package for the second version of the sample.
-      See `nRF Util mcu-manager serial`_ for details on how to use the command to perform the DFU.
+
+   a. Bluetooth firmware loader:
+
+      Open the `nRF Connect Device Manager`_ mobile app to perform DFU over Bluetooth® LE.
+
+      * The firmware loader advertises itself as *FW loader* and accepts MCUmgr image upload.
+      * Send the generated update package for the second version of the sample.
+        See :ref:`ug_nrf54l_developing_ble_fota_steps_testing` for details on how to use the mobile app to perform the DFU.
+
+   b. USB CDC ACM serial firmware loader:
+
+      Use `nRF Util mcu-manager serial` to perform DFU over serial port.
+
+      * Send the generated update package for the second version of the sample.
+        See `nRF Util mcu-manager serial`_ for details on how to use the command to perform the DFU.
+
 #. Verify that the printed build time corresponds to the new version once the update is complete and the device reboots into the main application.
 
 Dependencies