doc: dts: add guide to phandles

Phandles, specifier spaces, and cell names are simultaneously
extremely common and woefully underdocumented. Address that by:

- reworking our existing documentation on these subjects in
  bindings-syntax.rst, fixing missing information in the
  property syntax template as well

- adding a standalone guide which describes how all the pieces
  fit together, providing a bridge for the gap between
  DTS/bindings and C APIs

My goal is not to eventually make this a comprehensive place
where *all* specifier spaces are documented. It would be better (more
scalable, more discoverable) to improve the individual API pages to
cover the devicetree-related conventions that apply in each case.
That's a problem for someone else and another day, but we do need a
few concrete references in the DTS guides to keep the motivation
clear.

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