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/en/api-reference/peripherals/spi_flash/index.rst

@@ -175,6 +175,10 @@ Note that since memory mapping happens in pages, it may be possible to read data
 SPI Flash Implementation
 ------------------------
 
+.. note::
+
+    The header files in ``components/spi_flash/include/esp_flash_chips/`` directory are **semi-public** - they are intended for expert users who need to implement custom chip drivers for unsupported flash chips, but they are **not considered stable API** and may change without notice. For most use cases, you should use the public APIs in ``esp_flash.h`` instead.
+
 The ``esp_flash_t`` structure holds chip data as well as three important parts of this API:
 
 1. The host driver, which provides the hardware support to access the chip;
@@ -193,7 +197,7 @@ You can also implement your own host driver, even with the GPIO. As long as all
 Chip Driver
 ^^^^^^^^^^^
 
-The chip driver, defined in ``spi_flash_chip_driver.h``, wraps basic functions provided by the host driver for the API layer to use.
+The chip driver, defined in ``esp_flash_chips/spi_flash_chip_driver.h``, wraps basic functions provided by the host driver for the API layer to use.
 
 Some operations need some commands to be sent first, or read some status afterwards. Some chips need different commands or values, or need special communication ways.
 

docs/en/api-reference/peripherals/spi_flash/spi_flash_override_driver.rst

@@ -149,7 +149,12 @@ Generic Flash Driver
 The flash driver in the application is used to read, write, erase, and save data. It also supports some advanced features like OTA. Below is a guide on how to customize the driver for your specific flash model.
 
 - Step 1: The last item of `default_registered_chips` should be the `generic chip driver <https://github.com/espressif/esp-idf/blob/master/components/spi_flash/spi_flash_chip_generic.c>`_. If your flash chip does not match any of the chip drivers listed above, it will use the generic driver. Check for any differences in behavior between your flash and the generic driver, including but not limited to different commands, dummy cycles, data bytes, and status registers.
-- Step 2: If you have found something different from the generic driver, you need to implement your own chip driver. Create a new file named ``spi_flash_chip_<vendor>.c`` to implement the specific behavior, and copy the ``esp_flash_chip_generic`` structure into it as a starting point. Remember to include ``spi_flash_chip_generic.h``. Here is an example `esp_flash_nor <https://github.com/espressif/esp-flash-drivers/tree/main/esp_flash_nor/>`_.
+- Step 2: If you have found something different from the generic driver, you need to implement your own chip driver. Create a new file named ``spi_flash_chip_<vendor>.c`` to implement the specific behavior, and copy the ``esp_flash_chip_generic`` structure into it as a starting point. Remember to include ``esp_flash_chips/spi_flash_chip_generic.h``. Here is an example `esp_flash_nor <https://github.com/espressif/esp-flash-drivers/tree/main/esp_flash_nor/>`_.
+
+  .. note::
+
+      The chip driver header files are located in the ``esp_flash_chips/`` directory (e.g., ``components/spi_flash/include/esp_flash_chips/``). These headers are **semi-public** - they are intended for expert users who need to implement custom chip drivers, but they are **not considered stable API** and may change without notice.
+
 - Step 3: Implement the functions with difference and point to them from the ``spi_flash_chip_t``. Note: if some behavior of your flash is the same as the generic one, retain the generic driver functions without customization. Only implement the parts that differ. Here is an example:
 
 .. important::