docs(storage): clarify example READMEs and when to use each

Update storage example docs so FATFS getting_started and wear_levelling
are clearly distinct, refresh the storage index paths, and add
"When to use this example" sections across storage examples.

examples/storage/nvs/nvsgen/README.md

@@ -52,3 +52,8 @@ I (387) example: Reading values from NVS done - all OK
 ```
 
 The logic of the example is contained in a [single source file](./main/nvsgen_example_main.c), and it should be relatively simple to match points in its execution with the log outputs above.
+
+## When to use this example
+
+- You want to pre-populate an NVS partition at build time from a CSV file and flash it with the app.
+- You need factory/default configuration baked into NVS without writing it from firmware.