Syntax highlight all the DTS fragments and add some more explanatory
text.
Split the content about important properties into its own section, and
add a similar section about unit addresses.
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Re-work the introductory sections of the devicetree documentation,
adding several figures and cross-references to other useful parts of
the documentation.
Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>