doc: reference: add devicetree bindings content

Add generated documentation content for known devicetree bindings
using the Binding abstraction which was just added to edtlib. This
works similarly to the way Kconfig content is generated, so extract a
bit of common helper code for doing that out and rename the relevant
files to keep the distinction clear.

Make the documentation build system respect a preset DTS_ROOT. In this
way, out of tree bindings can be added to the generated content by
telling the documentation build system where to find them, identically
to how out of tree bindings can be added to a Zephyr application.

Similarly, make the output directory configurable.

Fixes: #28865
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>