doc: finish reworking the board porting guide

Now that the west parts of this file are in better shape, it seems a
shame not to flesh out the rest a bit more.

It's been a while since we looked at this document, as it's still
referring to boards (like Arduino 101) that are no longer supported by
Zephyr, and is generally lacking in concrete, step-by-step advice
for going from zero to working board.

Let's fix that.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>