docs: dt: convert writing-bindings.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Mark literal blocks as such;
- Add it to bindings/index.rst.

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