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>
Martí Bolívar