doc: devicetree: overhaul bindings guide

The binding-template.yaml file has grown organically into something
that's out of control.

It makes too many 'see above' and 'see below' references to be read
comfortably, and we can't cross reference from YAML.

There are also many example DTS and YAML fragments scattered about in
comments, which cannot be syntax highlighted properly.

Fix that by overhauling the documentation into bindings.rst in the DT
guide. This will let us link to individual sections when answering
questions, allows us to cross-reference and use '.. code-block::',
etc.

A couple of things need to go to other pages.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>