doc: dts/macros.rst improvements

Syntax highlight all the DTS fragments, add more internal
cross-referencing to making jumping around the HTML easier, and tweak
the language, filling in a missing piece here and there.

Fix a couple of DTS syntax errors caught by adding highlighting.

Add an ABNF grammar for the macros generated by DT, along with
some comments about why the current grammar is not ideal from a
generality point of view.

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