doc: split devicetree docs into multiple pages

The one page on devicetree is too long. Split it into multiple pages
to make it easier to digest and more squintable. This is basically
just moving content around; minimal changes have been made apart from
redoing some transitions and adding a couple of introductory paragraphs.

Rename the 'device-tree' Sphinx :ref: target while we are here.

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