doc: add pages about data and key storage

Added new pages about data storage and key storage.
Added images related to data storage.
NCSDK-32510.

Signed-off-by: Grzegorz Ferenc <Grzegorz.Ferenc@nordicsemi.no>
Co-authored-by: Sigurd Hellesvik <Sigurd.Hellesvik@nordicsemi.no>

Author: 2 people

doc/nrf/app_dev.rst

@@ -24,6 +24,7 @@ For detailed integration instructions of specific technologies and components, c
    app_dev/programming
    app_dev/companion_components
    app_dev/bootloaders_dfu/index
+   app_dev/data_storage
    app_dev/device_guides/nrf91/index
    app_dev/device_guides/nrf70/index
    app_dev/device_guides/nrf54l/index