new_variant: add doc section for Zork

Add a section showing the process for creating a Zork board, using
either Trembyle or Dalboz as the reference board.

BUG=b:157100630
TEST=N/A

Change-Id: I2b1e1cc5d0a1af60390aa37c33405898ca07829c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2210872
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
Commit-Queue: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Raul E Rangel <rrangel@chromium.org>