doc: dts: fix reference table of contents generation

Commit 6e9a43be ("doc: move DTS under Build/Configuration systems
section") made it so that the internal organization of the API
reference page is no longer visible to users. This means users have to
scroll around looking for macros they need, instead of being able to
look for a section that looks relevant and reading just that.

Fix it by restoring the independent toctree for DT reference material
with an appropriate maxdepth, and adding a local table of contents in
the reference page.

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