docs-rst: add input docs at main index and use kernel-figure

The input subsystem documentation got converted into ReST.

Add it to the main documentation index and use kernel-figure
for the two svg images there.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>