doc: update 91 Series DFU docs to reflect changes with sysbuild

Changes to Fota docs due to moving from child/parent images
build system to the new sysbuild build system.

Signed-off-by: divya pillai <[email protected]>

Author: divipillai

doc/nrf/app_dev/device_guides/nrf91/nrf91_building.rst

@@ -63,8 +63,8 @@ To perform a FOTA update, complete the following steps:
       This step does not apply for updates of the modem firmware.
       You can download delta patches and full binaries of the modem firmware from the `nRF9151 product website (compatible downloads)`_, `nRF9161 product website (compatible downloads)`_, or `nRF9160 product website (compatible downloads)`_, depending on the SiP you are using.
 
-   |fota_upgrades_building|
-   The :file:`app_update.bin` file must be uploaded to the server.
+   To create a binary file for an application update, build the application with the :kconfig:option:`SB_CONFIG_BOOTLOADER_MCUBOOT` option enabled.
+   The :file:`build/<app_name>/zephyr/zephyr.signed.bin` file must be uploaded to the server.
 
    To create binary files for a bootloader upgrade, make sure that the Kconfig options :kconfig:option:`CONFIG_SECURE_BOOT` and :kconfig:option:`CONFIG_BUILD_S1_VARIANT` are enabled and build MCUboot as usual.
    The build will create a binary file for each variant of the upgradable bootloader, one for each bootloader slot.

doc/nrf/shortcuts.txt

@@ -83,8 +83,6 @@
 .. |fota_upgrades_def| replace:: You can update the firmware of the device over the air, thus without a wired connection.
    Such an update is called a FOTA (firmware over-the-air) update.
 
-.. |fota_upgrades_building| replace:: To create a binary file for an application update, build the application with the :kconfig:option:`CONFIG_BOOTLOADER_MCUBOOT` option enabled.
-
 .. ### VSC shortcuts
 
 .. |vsc_extension_description| replace:: The |nRFVSC| is a complete IDE for developing applications compatible with the |NCS|.