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/partition_api/partition_find/README.md

@@ -8,6 +8,10 @@ These functions return the matching partitions given a set of constraints. By co
 properties that returned results should match - notably partition type, subtype and label/name. In the case of `esp_partition_find_first`, the first matching instance is returned;
 for `esp_partition_find`, an iterator is returned iterating over matching instances.
 
+## When to use this example
+
+- You need to locate partitions by type/subtype/label at runtime (`esp_partition_find` / `esp_partition_find_first`).
+
 # Example Flow
 
 The example uses a [custom partition table](./partitions_example.csv). The first part uses `esp_partition_find_first` to search for partition instances

examples/storage/partition_api/partition_mmap/README.md

@@ -5,6 +5,10 @@
 
 This example demonstrates how to use `esp_partition_mmap` to configure MMU and map a partition into memory address space for read operations.
 
+## When to use this example
+
+- You want to read a flash partition as if it were memory (memory-mapped), e.g. for zero-copy access to large read-only data.
+
 # Example Flow
 
 The example uses a [custom partition table](./partitions_example.csv), with a data partition `storage` used for demonstration. Before mapping this partition to memory,

examples/storage/partition_api/partition_ops/README.md

@@ -5,6 +5,10 @@
 
 This example demonstrates how to perform partition read, write and erase operations using API functions `esp_partition_read`, `esp_partition_write` and `esp_partition_erase`.
 
+## When to use this example
+
+- You want raw read/write/erase access to a flash partition without a filesystem on top.
+
 # Example Flow
 
 The example uses a [custom partition table](./partitions_example.csv), with a data partition `storage` used as the demo partition. For the most part the example code is well-commented so users should be able to follow along the code easily. Nevertheless, this section provides an overview of the code flow.