doc: build overview: Improve diagram, fix errors/stale info, misc.

Diagram and text improvements:

 - Redraw the configuration phase diagram to better reflect the actual
   logic. Remove some misleading arrows, like from devicetree.h to
   Kconfig. Kconfig uses the devicetree scripts directly.

 - After the old devicetree scripts were removed in commit c8c35f76
   ("scripts: dts: Remove deprecated extract_dts_includes.py script"),
   the dtc compiler is only run to catch any high-level warnings and
   errors from it. The output is unused.

   Update the diagram and descriptions to explain how dtc is used.

 - Mention kconfigfunctions.py and explain better how devicetree and
   Kconfig interact

 - Clarify that 'cpp' is the C preprocessor. People often confuse it
   with C++.

 - Fix a typo'd devicetree_fixups.h in the text

 - Use the :file: role for files instead of italic text

 - Add links to the devicetree and Kconfig sections of the manual, and
   use the :zephyr_file: role to turn more files into direct links

 - Make the text generic re. Make vs. Ninja

 - Lots of other minor tweaks and clarifications

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