doc: dts: add design goals defining the scope of DT

Add a page describing the high-level design goals for how Zephyr
should use DT, with examples and counter-examples from current
practice.

Add a TBD section for code generation. It's not clear (to me at least)
where the discussion on that has landed.

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