docs: split up the driver book

We don't need to keep it as a single large file anymore; split it up so
that it is easier to manage and the individual sections can be read
directly as plain files.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/input.rst

@@ -0,0 +1,51 @@
+Input Subsystem
+===============
+
+Input core
+----------
+
+.. kernel-doc:: include/linux/input.h
+   :internal:
+
+.. kernel-doc:: drivers/input/input.c
+   :export:
+
+.. kernel-doc:: drivers/input/ff-core.c
+   :export:
+
+.. kernel-doc:: drivers/input/ff-memless.c
+   :export:
+
+Multitouch Library
+------------------
+
+.. kernel-doc:: include/linux/input/mt.h
+   :internal:
+
+.. kernel-doc:: drivers/input/input-mt.c
+   :export:
+
+Polled input devices
+--------------------
+
+.. kernel-doc:: include/linux/input-polldev.h
+   :internal:
+
+.. kernel-doc:: drivers/input/input-polldev.c
+   :export:
+
+Matrix keyboards/keypads
+------------------------
+
+.. kernel-doc:: include/linux/input/matrix_keypad.h
+   :internal:
+
+Sparse keymap support
+---------------------
+
+.. kernel-doc:: include/linux/input/sparse-keymap.h
+   :internal:
+
+.. kernel-doc:: drivers/input/sparse-keymap.c
+   :export:
+