docs: Adds CI tests for the doc building process.

Adds test_sphinx_idf_extensions.py and test_docs.py for testing the documentation building process.
Both are run as a part of the host_test stage.

Closes IDF-1648

docs/test/en/conf.py

@@ -0,0 +1,33 @@
+# -*- coding: utf-8 -*-
+#
+# English Language RTD & Sphinx config file
+#
+# Uses ../conf_common.py for most non-language-specific settings.
+# Importing conf_common adds all the non-language-specific
+# parts to this conf module
+try:
+    from conf_common import *  # noqa: F403,F401
+except ImportError:
+    import sys
+    import os
+    sys.path.insert(0, os.path.abspath('../..'))
+    from conf_common import *  # noqa: F403,F401
+
+# General information about the project.
+project = u'ESP-IDF Programming Guide'
+copyright = u'2016 - 2020, Espressif Systems (Shanghai) CO., LTD'
+
+# The language for content autogenerated by Sphinx. Refer to documentation
+# for a list of supported languages.
+language = 'en'
+html_copy_source = False
+
+
+html_logo = None
+latex_logo = None
+html_static_path = []
+
+conditional_include_dict = {'esp32':["esp32_page.rst"],
+                            'esp32s2':["esp32s2_page.rst"],
+                            'SOC_BT_SUPPORTED':["bt_page.rst"],
+                            }