refactor(spi_flash): reorganize header files and improve API encapsulation

This commit reorganizes SPI flash header files to better reflect their
visibility and intended usage:

1. Rename `esp_flash_port/` to `esp_flash_chips/`:
   - Better reflects that these headers are for chip driver implementations
   - All chip driver headers moved to `esp_flash_chips/` directory
   - Added README.md explaining semi-public nature of these headers

2. Move internal headers to `esp_private/`:
   - `esp_flash_internal.h` -> `esp_private/esp_flash_internal.h`
   - `memspi_host_driver.h` -> `esp_private/memspi_host_driver.h`

3. Move chip driver related headers to `esp_flash_chips/`:
   - `esp_private/esp_flash_types.h` -> `esp_flash_chips/esp_flash_types.h`
   - `spi_flash/spi_flash_defs.h` -> `esp_flash_chips/spi_flash_defs.h`
   - `spi_flash_override.h` -> `esp_flash_chips/spi_flash_override.h`
   - All `spi_flash_chip_*.h` headers moved to `esp_flash_chips/`

4. Code improvements:
   - Remove unused includes (e.g., `spi_flash_override.h` from `cache_utils.c`)
   - Use public API `esp_flash_get_size()` instead of direct member access
   - Add `esp_flash_is_quad_mode` to linker.lf for IRAM placement

5. Documentation updates:
   - Add README.md in `esp_flash_chips/` explaining semi-public headers
   - Update programming guide with warnings about internal headers
   - Update both English and Chinese documentation

6. Update all references across the codebase:
   - Update includes in `spi_flash` component
   - Update `bootloader_support`, `app_update`, `esp_tee`, `espcoredump`
   - Update example projects

Breaking changes:
- Headers moved to new locations require include path updates
- `custom_flash_driver` example temporarily disabled until external
  components are updated

docs/zh_CN/api-reference/peripherals/spi_flash/index.rst

@@ -175,6 +175,10 @@ flash 在 {IDF_TARGET_CACHE_SIZE} 页进行映射。内存映射硬件既可将
 SPI flash 实现
 --------------
 
+.. note::
+
+    实现部分的头文件（例如 ``esp_flash_chips/``、``esp_private/``）是**内部**的 - 它们面向需要为不支持的 flash 芯片实现自定义芯片驱动的专家用户，但**不被视为稳定 API**，可能会在不通知的情况下更改。对于大多数用例，您应该使用 ``esp_flash.h`` 中的公共 API。
+
 ``esp_flash_t`` 结构体包含芯片数据和该 API 的三个重要部分：
 
 1. 主机驱动，为访问芯片提供硬件支持；
@@ -193,7 +197,7 @@ SPI flash 实现
 芯片驱动
 ^^^^^^^^^^^
 
-芯片驱动在 ``spi_flash_chip_driver.h`` 中进行定义，并将主机驱动提供的基本函数进行封装以供 API 层使用。
+芯片驱动在 ``esp_flash_chips/spi_flash_chip_driver.h`` 中进行定义，并将主机驱动提供的基本函数进行封装以供 API 层使用。
 
 有些操作需在执行前先发送命令，或在执行后读取状态，因此有些芯片需要不同的命令或值以及通信方式。