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

components/esp_rom/esp32s31/include/esp32s31/rom/esp_flash.h

@@ -16,7 +16,6 @@ extern "C" {
    just adds ROM-specific parts
 */
 
-struct spi_flash_chip_t;
 typedef struct esp_flash_t esp_flash_t;
 
 /* Structure to wrap "global" data used by esp_flash in ROM */