doc: dts: split up bindings docs

The bindings.rst page has gotten too big. Split it up into
sub-documents to improve readability and restore maintainability.

As part of this effort, move the /zephyr,user node documentation into
its own page in the guide. This page has proven extremely difficult
for users to spot in my experience, and it's meant as a convenience,
so let's make the documentation more convenient as well.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>