doc: update board porting guide to match new hardware model

Update board porting guide to the design changes in the new hw model.

New section added:
- board.yml file description

Updated sections:
- Board on SoCs example table updated to new format and extra examples
  added.
- Board directory description and its files
- Writing devicetree file section updated to match new lookup patterns
- Writing Kconfig file section updated to match new lookup patterns
- Board revision handling updated

In getting started guide a note has been added with short description
of the new board and board identifier scheme.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>