feat(storage): Add generic partition BDL example

Add an example under examples/storage/generic_partition_bdl that
partitions a raw block device at run time using only the Block Device
Layer (BDL) interface.

The example obtains a whole-disk BDL (SPI flash data partition by
default, or an SD/eMMC card via menuconfig), writes an MBR partition
table onto it with the esp_ext_part_tables managed component (fetched
via idf_component.yml), then creates a generic-partition BDL for each
MBR entry with esp_blockdev_generic_partition_get() and mounts FATFS on
the FAT slice and LittleFS on the LittleFS slice.

docs/zh_CN/api-reference/storage/blockdev.rst

@@ -157,6 +157,11 @@
 
 .. _blockdev-apis:
 
+应用示例
+--------
+
+- :example:`storage/generic_partition_bdl` 演示了如何仅使用块设备层在运行时对裸块设备进行分区：将 MBR 分区表写入整盘 BDL（SPI flash 数据分区或 SD/eMMC 卡），并在通用分区 BDL 切片上挂载 FATFS 和 LittleFS。
+
 API 参考
 --------