[nrf fromlist] doc: zms: add documentation for ZMS

This adds the documentation for the Zephyr Memory Storage system.

Upstream PR: zephyrproject-rtos/zephyr#77930

Signed-off-by: Riadh Ghaddab <[email protected]>
(cherry picked from commit a5db5ee0e5f1b43fb00fe6f1089f542b69cfbb1a)

Author: rghaddab

doc/services/storage/index.rst

@@ -7,6 +7,7 @@ Storage
    :maxdepth: 1
 
    nvs/nvs.rst
+   zms/zms.rst
    disk/access.rst
    flash_map/flash_map.rst
    fcb/fcb.rst