doc: write documentation about DT-based device instantiation

Add more HOWTO information for the two current devicetree-based device
instantiation styles, and a bit more information on how to create
devices that depend on others.

Point to this from the Kconfig tips page, since it is meant as a
replacement for existing Kconfig practice.

Update macros.bnf.

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