doc: Add section on how to add an external machine

This patch adds a section detailing how to add a yaml file for an
external platform and how to build images for it.

Issue-Id: SCM-3031
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>
Change-Id: I3be02bf39de84cd89f2c9a8c38d902f8ba74f861