doc: guides: dts improvements

People seem to miss the troubleshooting page a lot, since nobody ever
mentions that they tried its steps and they didn't work. Promote it to
a top level page to make it easier to see. Add cross-references
between the two to keep them well linked together.

Add a couple of extra links to the bindings related HOWTO while we are
here, now that the bindings docs are a bit better off than they were
when this content was originally written.

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