doc: dts: improve bindings-upstream

Splitting up the 'general rules' section into subsections makes it
possible to link directly to a particular rule. This is useful when
pointing out an issue during code review, sharing with another
colleague, etc. Adding a local table of contents makes the page
skimmable (it's buried too deeply in the toctree to have sections
listed in the HTML sidebar).

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