feat(examples): add Ethernet sublayer example and update PTP

Add a dedicated sublayer example under examples/ethernet and enable
sublayer usage in the PTP example.

docs/en/api-reference/network/esp_eth.rst

@@ -683,6 +683,20 @@ The majority of PHY management functionality required by the ESP-IDF Ethernet dr
 
 Once you finish the new custom PHY driver implementation, consider sharing it among other users via `ESP Component Registry <https://components.espressif.com/>`_.
 
+.. _ethernet-sublayer:
+
+Ethernet Sublayer
+^^^^^^^^^^^^^^^^^
+
+The Ethernet sublayer is an optional layer between one physical Ethernet driver (``esp_eth_handle_t``) and one or more ``esp_netif`` instances. It owns the driver RX input path, distributes Ethernet events to attached interfaces, and can perform mid-path frame processing such as 802.1Q VLAN multiplexing and demultiplexing.
+
+.. note::
+    The Ethernet sublayer is an **experimental** feature. Enable :menuitem:`CONFIG_IDF_EXPERIMENTAL_FEATURES` and :menuitem:`CONFIG_ETH_SUBLAYER_SUPPORT` to use it. The API is under active development and may change in future ESP-IDF releases without a deprecation period.
+
+For architecture details, configuration, and usage guidance, see :component_file:`esp_eth/src/sublayer/README.md`.
+
+A working demonstration is available in :example:`ethernet/sublayer`, which shares one Ethernet driver between an untagged ``esp_netif`` and a tagged VLAN ``esp_netif``.
+
 .. ---------------------------- API Reference ----------------------------------
 
 API Reference

docs/zh_CN/api-reference/network/esp_eth.rst

@@ -683,6 +683,20 @@ ESP-IDF 以太网驱动程序所需的大部分 PHY 管理功能都已涵盖在
 
 实现新的自定义 PHY 驱动程序后，你可以通过 `乐鑫组件注册表 <https://components.espressif.com/>`_ 将驱动分享给其他用户。
 
+.. _ethernet-sublayer:
+
+以太网子层
+^^^^^^^^^^
+
+以太网子层是位于一个物理以太网驱动程序（``esp_eth_handle_t``）与一个或多个 ``esp_netif`` 实例之间的可选层。它负责拥有驱动程序的 RX 输入路径，将以太网事件分发给所连接的接口，并可执行路径中的帧处理，例如 802.1Q VLAN 复用与解复用。
+
+.. note::
+    以太网子层是一项**实验性**功能。需启用 :menuitem:`CONFIG_IDF_EXPERIMENTAL_FEATURES` 和 :menuitem:`CONFIG_ETH_SUBLAYER_SUPPORT` 后方可使用。该 API 仍在积极开发中，未来 ESP-IDF 版本中可能会在不经过弃用周期的情况下发生变更。
+
+有关架构细节、配置和使用指南，请参阅 :component_file:`esp_eth/src/sublayer/README.md`。
+
+可参考 :example:`ethernet/sublayer` 中的示例，该示例演示了如何在一个未打标签的 ``esp_netif`` 和一个带标签的 VLAN ``esp_netif`` 之间共享同一个以太网驱动程序。
+
 .. ---------------------------- API Reference ----------------------------------
 
 API 参考