docs: add language link to easily change to Chinese version

docs/en/index.rst

@@ -1,8 +1,11 @@
 ESP-IDF Programming Guide
 =========================
+:link_to_translation:`zh_CN:[中文]`
 
 This is the documentation for Espressif IoT Development Framework (`esp-idf <https://github.com/espressif/esp-idf>`_). ESP-IDF is the official development framework for the `ESP32 <https://espressif.com/en/products/hardware/esp32/overview>`_ chip.
 
+The documentation has different language versions (:link_to_translation:`en:English`, :link_to_translation:`zh_CN:中文版`, :doc:`How to switch between languages? <languages>`). However, please refer to the English version if there is any discrepancy.
+
 ==================  ==================  ==================
 |Get Started|_      |API Reference|_    |H/W Reference|_
 ------------------  ------------------  ------------------
@@ -44,5 +47,6 @@ This is the documentation for Espressif IoT Development Framework (`esp-idf <htt
    Resources <resources>
    Copyrights <COPYRIGHT>
    About <about>
+   [语言/Languages] <languages>
    
 * :ref:`genindex`

docs/en/languages.rst

@@ -0,0 +1,19 @@
+Switch Between Languages/切换语言
+=================================
+
+The documentation now has two language versions, however, please refer to the English version if there is any discrepancy.
+
+本文档现在有两种语言的版本，如有出入请以英文版本为准。
+
+- English/英文
+- Chinese/中文
+
+You can easily changed from one language to another by the panel on the sidebar like below. Just click on the **Read
+the Docs** title button on the left-bottom conner if it is folded.
+
+如下图所示，你可使用边栏的面板进行语言的切换。如果该面板被折叠，点击左下角 **Read the Docs** 标题按钮来显示它。
+
+.. image:: /../_static/choose_version.png
+
+
+

docs/zh_CN/index.rst

@@ -1,8 +1,11 @@
 ESP-IDF 编程指南
 =========================
+:link_to_translation:`en:[English]`
 
 这是乐鑫 IoT 开发开发框架 (`esp-idf <https://github.com/espressif/esp-idf>`_) 的文档. ESP-IDF 是 `ESP32 <https://www.espressif.com/zh-hans/products/hardware/esp32/overview>`_ 芯片的官方开发框架.
 
+本文档提供不同语言的翻译版本 (:link_to_translation:`en:English`, :link_to_translation:`zh_CN:中文版`, :doc:`如何切换语言？ <languages>`), 如有出入请以英文版本为准。
+
 ==================  ==================  ==================
 |快速入门|_          |API 参考|_         |H/W 参考|_
 ------------------  ------------------  ------------------
@@ -44,5 +47,7 @@ ESP-IDF 编程指南
    相关资源 <resources>
    版权 <COPYRIGHT>
    关于 <about>
+   [Languages/语言] <languages>
+
 
 * :ref:`genindex`

docs/zh_CN/languages.rst

@@ -0,0 +1 @@
+.. include:: ../en/languages.rst