docs: split spi_flash api-reference documents to peripherals folder

docs/en/api-reference/storage/index.rst

@@ -3,6 +3,21 @@ Storage API
 
 :link_to_translation:`zh_CN:[中文]`
 
+
+This section contains reference of the high-level storage APIs. They are based on low-level drivers such as SPI Flash, SD/MMC.
+
+- :doc:`Partitions API <partition>` allow block based access to SPI Flash according to the :doc:`Partition Table </api-guides/partition-tables>`.
+- :doc:`Non-Volatile Storage library (NVS) <nvs_flash>` implements a fault-tolerant wear-levelled key-value storage in SPI NOR Flash.
+- :doc:`Virtual File System (VFS) <vfs>` library provides an interface for registration of file system drivers. SPIFFS, FAT and various other file system libraries are based on the VFS.
+- :doc:`SPIFFS <spiffs>` is a wear-levelled file system optimized for SPI NOR Flash, well suited for small partition sizes and low throughput
+- :doc:`FAT <fatfs>` is a standard file system which can be used in SPI Flash or on SD/MMC cards
+- :doc:`Wear Levelling <wear-levelling>` library implements a flash translation layer (FTL) suitable for SPI NOR Flash. It is used as a container for FAT partitions in Flash.
+
+.. note::
+    It's suggested to use high-level APIs (``esp_partition`` or file system) instead of low-level driver APIs to access the SPI NOR Flash.
+
+    Due to the restriction of NOR Flash and ESP hardware, accessing the main flash will affect the performance of the whole system. See :doc:`SPI Flash Documents </api-reference/peripherals/spi_flash/spi_flash>` to learn more about the limitations.
+
 .. toctree::
     :maxdepth: 1
 
@@ -12,11 +27,10 @@ Storage API
     nvs_partition_gen.rst
     nvs_partition_parse.rst
     sdmmc
-    spi_flash
+    partition
     spiffs
     vfs
     wear-levelling
 
-
-
 Code examples for this API section are provided in the :example:`storage` directory of ESP-IDF examples.
+