Add Sphinx API doc generation (#3029)

* add's sphinx project to docs/ dir
* include core/helpers autodocs for API reference

docs/source/api/event.rst

@@ -0,0 +1,20 @@
+.. _helpers_event_module:
+
+:mod:`homeassistant.helpers.event`
+----------------------------------
+
+.. automodule:: homeassistant.helpers.event
+
+.. autofunction:: track_state_change
+
+.. autofunction:: track_point_in_time
+
+.. autofunction:: track_point_in_utc_time
+
+.. autofunction:: track_sunrise
+
+.. autofunction:: track_sunset
+
+.. autofunction:: track_utc_time_change
+
+.. autofunction:: track_time_change