Add Sphinx API doc generation (#3029)

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

docs/source/api/core.rst

@@ -0,0 +1,18 @@
+.. _core_module:
+
+:mod:`homeassistant.core`
+-------------------------
+
+.. automodule:: homeassistant.core
+
+.. autoclass:: Config
+    :members:
+
+.. autoclass:: EventBus
+    :members:
+
+.. autoclass:: StateMachine
+    :members:
+
+.. autoclass:: ServiceRegistry
+    :members: