move api reference core index page to actual index route

this improves how the nav builds -- generating a clickable header
instead of an expand-header that has a first item with the same name

Author: bosh

docs/code/api_reference/api_reference_core.md

@@ -0,0 +1,22 @@
+# Core Components
+
+Core MPF machine components, accessible to programmers at `self.machine.*name*`. For example, the ball controller is at `self.machine.ball_controller`, the event manager is `self.machine.events`, etc.
+
+* [auditor](auditor.md)
+* [ball_controller](ball_controller.md)
+* [bcp](bcp.md)
+* [device_manager](device_manager.md)
+* [events](events.md)
+* [info_lights](info_lights.md)
+* [light_controller](light_controller.md)
+* [machine](machine.md)
+* [mode_controller](mode_controller.md)
+* [placeholder_manager](placeholder_manager.md)
+* [platform_controller](platform_controller.md)
+* [service](service.md)
+* [settings](settings.md)
+* [show_controller](show_controller.md)
+* [switch_controller](switch_controller.md)
+* [switch_player](switch_player.md)
+* [text_ui](text_ui.md)
+* [twitch_bot](twitch_bot.md)

docs/code/api_reference/core/index.md

@@ -2,7 +2,7 @@
 
 MPF's API reference is broken into several categories. All of it is presented in the way that the modules and classes are actually used in MPF.
 
-* [Core Components](api_reference_core.md): MPF core components.
+* [Core Components](core/index.md): MPF core components.
 
 * [Devices](api_reference_device.md): MPF devices, including physical devices like flippers, ball devices, switches, lights, etc. as well as logical devices like ball saves, extra balls, multiballs, etc.
 

docs/code/api_reference/index.md

@@ -1358,7 +1358,7 @@ nav:
     - API Reference:
       - code/api_reference/index.md
       - Core:
-        - code/api_reference/api_reference_core.md
+        - code/api_reference/core/index.md
         - code/api_reference/core/auditor.md
         - code/api_reference/core/ball_controller.md
         - code/api_reference/core/bcp.md