documentation: update MCUBoot docs for local update agent example

Updates the example documentation for a new example: local update agent.
This example allows the user to update MCUBoot image from a locally stored file.

Signed-off-by: Filipe Cavalcanti <filipe.cavalcanti@espressif.com>

Documentation/applications/examples/mcuboot/index.rst

@@ -3,10 +3,10 @@
 ============================
 
 ``swap_test``
--------------
+=============
 
 Description
-~~~~~~~~~~~
+-----------
 
 MCUboot Swap Image is an application to demonstrate firmware upgrade using
 internal flash memory. It simulate MCUboot API steps to switch between two
@@ -17,7 +17,7 @@ After application is build and ``nuttx.bin`` be generated, the binary must be
 signed. Consult your board documentation page to get instructions how to do it.
 
 How to build and flash
-......................
+----------------------
 
 First step is build your board configuration using ``mcuboot-loader`` as target.
 That create the bootloader itself. The ``nuttx.bin`` must be flash as usual.
@@ -37,7 +37,7 @@ More instructions about how to sign and flash can be found at board documentatio
 page.
 
 Running swap image test
-.......................
+-----------------------
 
 Open you terminal and reboot your board. You can see a similar output as below.
 You can check builtin apps using command ``?``::
@@ -129,3 +129,109 @@ Sixth step (Reboot and confirm V2 image)::
 
 Conclusion, once we boot a newer image and confirm it MCUboot always run that
 image, unless you instruct it to swap again!
+
+``mcuboot_local_agent``
+=======================
+
+MCUBoot Local Update Agent is an application to demonstrate firmware upgrade
+using a binary file from local storage. Unlike remote update mechanisms, this
+example copies the firmware binary directly from local storage (such as SD Card,
+USB drive, or any mounted filesystem) to the secondary flash slot for MCUBoot
+to process during the next boot.
+
+This application provides a simple and reliable way to update firmware without
+requiring network connectivity or complex remote update infrastructure.
+
+Features:
+
+* Copy firmware binary from local storage to secondary flash slot
+* Progress indication during firmware copy
+* Automatic size validation (ensures firmware fits in secondary slot)
+* Flash area erasing and writing
+* Automatic boot marking and system restart
+
+Build and flash
+----------------------
+
+First, build your board configuration with MCUBoot, SD Card support and enable this tool under
+`Application Configuration → Examples → MCUboot Examples → MCUBoot Local Update Agent`.
+
+Flash the board normally.
+
+Generating the update binary
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To create a firmware binary for update:
+
+1. Configure the build to target the secondary slot::
+
+     make menuconfig
+     # Configure MCUBoot to build for secondary slot
+     # (specific configuration varies by architecture)
+
+2. Optionally modify the application (e.g., change MOTD) to visually identify
+   the updated image
+
+3. Build the update binary::
+
+     make
+
+4. The resulting binary should be copied to your local storage device (i.e. SD Card).
+   Consult your board documentation for the specific binary filename and
+   any required post-processing steps.
+
+Running the local update agent
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+1. Mount your storage device containing the firmware binary::
+
+     nsh> mount -t vfat /dev/mmcsd0 /mnt
+
+2. Verify the firmware file is accessible::
+
+     nsh> ls /mnt
+     /mnt:
+      nuttx.bin
+      readme.txt
+
+3. Run the update agent with the firmware file path::
+
+     nsh> mcuboot_local_agent /mnt/nuttx.bin
+
+   If no path is specified, it defaults to ``/mnt/sdcard/nuttx.bin``
+
+Example output
+~~~~~~~~~~~~~~
+
+When running the update agent, you should see output similar to::
+
+  nsh> mcuboot_local_agent /mnt/nuttx.bin
+  MCUBoot Local Update Agent
+  Firmware file: /mnt/nuttx.bin
+  Firmware file size: 1048576 bytes
+  Erasing secondary flash slot...
+  Copying firmware to secondary slot...
+  Progress: 4096/1048576 bytes [0%]
+  Progress: 8192/1048576 bytes [0%]
+  Progress: 12288/1048576 bytes [1%]
+  ...
+  Progress: 1044480/1048576 bytes [99%]
+  Progress: 1048576/1048576 bytes [100%]
+  Firmware copy completed successfully!
+  Firmware successfully copied to secondary slot!
+  Update scheduled for next boot. Restarting...
+  reboot status=0
+
+After reboot, MCUBoot will detect the new firmware in the secondary slot and
+perform the update. You should see MCUBoot messages indicating the swap process::
+
+  *** Booting MCUboot build v2.2.0-rc1 ***
+  ...
+  Primary image: magic=good, swap_type=0x1, copy_done=0x3, image_ok=0x1
+  Scratch: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
+  Boot source: primary slot
+  Image index: 0, Swap type: test
+  Starting swap using scratch algorithm.
+  ...
+  This is MCUBoot Updated Image!!
+  nsh>