doc: update the v3.0 migration guide

Adds CRACEN initialization info

Signed-off-by: Anna Wojdylo <[email protected]>

Author: annwoj

doc/nrf/releases_and_maturity/migration/migration_guide_3.0.rst

@@ -118,6 +118,24 @@ Bluetooth® LE legacy pairing
       Using Bluetooth LE legacy pairing introduces, among others, a risk of passive eavesdropping.
       Supporting Bluetooth LE legacy pairing makes devices vulnerable to downgrade attacks.
 
+CRACEN initialization
+---------------------
+
+.. toggle::
+
+   In the |NCS| versions 2.8.0 and 2.9.0, you must explicitly configure the CRACEN initialization.
+   It is done by adding the :kconfig:option:`CONFIG_CRACEN_LOAD_MICROCODE` Kconfig option to the image configuration.
+   This option allows to select the given image to initialize CRACEN.
+
+   However, from |NCS| 3.0.0, CRACEN is automatically initialized.
+   The new build configuration option (:kconfig:option:`SB_CONFIG_CRACEN_MICROCODE_LOAD_ONCE`) now controls this process at the sysbuild level.
+   When enabled, the build system automatically determines which image must handle the initialization of CRACEN.
+
+   Unlike in the |NCS| versions 2.8.0 and 2.9.0, where CRACEN initialization is disabled by default in the MCUboot configuration, CRACEN is initialized by the earliest bootloader by default in the |NCS| 3.0.0.
+   This change can lead to scenarios where CRACEN might be initialized twice or not initialized at all.
+   When migrating from the |NCS| v2.9.0 to v3.0.0, you must analyze which image is responsible for initializing CRACEN before and after the firmware update to ensure correct operation.
+   Make sure to adjust your bootloader or application upgrade path accordingly to avoid any issues related to CRACEN initialization.
+
 nRF54H20
 ========