docs: dt: Group DT docs into relevant sub-sections

The DT docs are currently all just lumped together in the doc build.
Let's organize the documents by kernel specifics, overlays and bindings.

As writing-schema.rst is about bindings, let's move it to the bindings
directory.

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20210325164713.1296407-6-robh@kernel.org

Documentation/devicetree/bindings/index.rst

 .. SPDX-License-Identifier: GPL-2.0
 
-===========
-Device Tree
-===========
-
 .. toctree::
    :maxdepth: 1
 
    ABI
-   submitting-patches
    writing-bindings
+   writing-schema
+   submitting-patches

Documentation/devicetree/index.rst

@@ -4,14 +4,26 @@
 Open Firmware and Devicetree
 =============================
 
+Kernel Devicetree Usage
+=======================
 .. toctree::
    :maxdepth: 1
 
    usage-model
-   writing-schema
+   of_unittest
+
+Devicetree Overlays
+===================
+.. toctree::
+   :maxdepth: 1
+
    changesets
    dynamic-resolution-notes
-   of_unittest
    overlay-notes
 
+Devicetree Bindings
+===================
+.. toctree::
+   :maxdepth: 1
+
    bindings/index