Input: docs - split input docs into kernel- and user-facing

Split input documentation into several groups: kernel- and user-facing, and
notes about individual device drivers. Move device drivers docs into a
separate subdirectory.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/input/input_uapi.rst

@@ -0,0 +1,21 @@
+.. include:: <isonum.txt>
+
+###################################
+Linux Input Subsystem userspace API
+###################################
+
+.. class:: toc-title
+
+        Table of Contents
+
+.. toctree::
+   :maxdepth: 2
+   :numbered:
+
+   input
+   event-codes
+   multi-touch-protocol
+   gamepad
+   ff
+   joydev/index
+   userio