Add warning to docs output if fastbuild

docs/en/index.rst

@@ -2,6 +2,12 @@ ESP-IDF Programming Guide
 =========================
 :link_to_translation:`zh_CN:[中文]`
 
+.. only:: fast_build
+
+    .. warning::
+
+        The function API documentation is excluded from the preview. Add docs_full label to the MR to generate the complete documentation.
+
 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, ESP32-S and ESP32-C Series SoCs <https://www.espressif.com/en/products/hardware/socs>`_.
 
 .. only:: html

docs/idf_extensions/run_doxygen.py

@@ -104,6 +104,7 @@ def convert_api_xml_to_inc(app, doxyfiles):
         if fast_build:
             with open(inc_file_path, 'w', encoding='utf-8') as inc_file:
                 inc_file.write('')
+            app.tags.add('fast_build')
 
 
 def get_doxyfile_input_paths(app, doxyfile_path):

docs/zh_CN/index.rst

@@ -2,6 +2,12 @@ ESP-IDF 编程指南
 =========================
 :link_to_translation:`en:[English]`
 
+.. only:: fast_build
+
+    .. warning::
+
+        The function API documentation is excluded from the preview. Add docs_full label to the MR to generate the complete documentation.
+
 这里是乐鑫 IoT 开发框架 (`esp-idf <https://github.com/espressif/esp-idf>`_) 的文档中心。ESP-IDF 是 `ESP32、ESP32-S 和 ESP32-C <https://www.espressif.com/en/products/hardware/socs>`_ 系列芯片的官方开发框架。
 
 .. only:: html