doc: release: 3.5: Add notes about ext2 fs

Add information about new support for ext2 and modified fs sample.

Signed-off-by: Franciszek Zdobylak <fzdobylak@antmicro.com>

doc/releases/release-notes-3.5.rst

@@ -283,6 +283,10 @@ Libraries / Subsystems
     :c:struct:`mgmt_group` when registering a transport. See
     :c:type:`smp_translate_error_fn` for function details.
 
+* File systems
+
+  * Added support for ext2 file system.
+
 HALs
 ****
 
@@ -313,5 +317,8 @@ Documentation
 Tests and Samples
 *****************
 
+* Created common sample for file systems (`fs_sample`). It originates from sample for FAT
+  (`fat_fs`) and supports both FAT and ext2 file systems.
+
 Known Issues
 ************