docs: dt: add an index.rst file for devicetree

There are some device tree documentation under
Documentation/devicetree. Add a top index file for it and
add the already-existing ReST file on it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>