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>

Documentation/devicetree/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=============================
+Open Firmware and Device Tree
+=============================
+
+.. toctree::
+   :maxdepth: 1
+
+   writing-schema

Documentation/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
 
 .. The Linux Kernel documentation master file, created by
    sphinx-quickstart on Fri Feb 12 13:51:46 2016.
@@ -46,6 +48,7 @@ platform firmwares.
    :maxdepth: 2
 
    firmware-guide/index
+   devicetree/index
 
 Application-developer documentation
 -----------------------------------