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

This adds the documentation for the Zephyr Memory Storage system.

Signed-off-by: Riadh Ghaddab <[email protected]>
(cherry picked from commit 3c4fcaa)
(cherry picked from commit 77095d5)

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