docs: update copyright date for docs automatically

docs/en/conf.py

@@ -14,9 +14,13 @@ except ImportError:
     sys.path.insert(0, os.path.abspath('../'))
     from conf_common import *  # noqa: F403,F401
 
+import datetime
+
+current_year = datetime.datetime.now().year
+
 # General information about the project.
 project = u'ESP-IDF Programming Guide'
-copyright = u'2016 - 2021, Espressif Systems (Shanghai) Co., Ltd'
+copyright = u'2016 - {}, Espressif Systems (Shanghai) Co., Ltd'.format(current_year)
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.