doc: zbus: add entries to the migration guide

ZBus had one break change related to the runtime observers'
configuration, and the VDED delivery sequence has changed. This commit
adds entries on the migration guide about the mentioned changes.

Signed-off-by: Rodrigo Peixoto <[email protected]>

Author: rodrigopex

doc/releases/migration-guide-3.5.rst

@@ -271,6 +271,17 @@ Required changes
 * The :kconfig:option:`CONFIG_RISCV_MTVEC_VECTORED_MODE` Kconfig option was renamed to
   :kconfig:option:`CONFIG_RISCV_VECTORED_MODE`.
 
+* ZBus runtime observers implementation now relies on the HEAP memory instead of a memory slab.
+  Thus, zbus' configuration (kconfig) related to runtime observers has changed. To keep your runtime
+  observers code working correctly, you need to:
+
+  - Replace the integer ``CONFIG_ZBUS_RUNTIME_OBSERVERS_POOL_SIZE`` with the boolean
+    :kconfig:option:`CONFIG_ZBUS_RUNTIME_OBSERVERS`;
+  - Set the HEAP size with the :kconfig:option:`CONFIG_HEAP_MEM_POOL_SIZE`.
+
+* The zbus VDED delivery sequence has changed. Check the :ref:`documentation<zbus delivery
+  sequence>` to verify if it will affect your code.
+
 Recommended Changes
 *******************