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