doc: dts: Add more details, implementation notes, and examples

Rewrite most of the 'Input and output files' section to add more details
and implementation notes, and to make some description more more
concrete:

 - Mention dtlib, edtlib, and gen_defines.py, and explain what they do

 - Add an example of how macros get generated from the devicetree, and
   explain the DT_<node>_<property> format of the generated identifiers.

   Merge the 'Include files generation' section into the 'Input and
   output files' section at the same time.

 - Explain that the base devicetree and the overlays just get
   concatenated, and why this works

 - Add more details on how dts_fixup.h files work

 - Mention that the C dtc compiler is only run to catch errors and
   warnings from it

 - Mention that the concatenated devicetree appears in
   zephyr/<BOARD>.dts.pre.tmp

 - Mention /include/, which is the native DTS mechanism for including
   other files

 - Misc. other minor tweaks

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>