ci/device-testing: Add new topic on device testing

* Overview of on-device testing.
* Testing Infrastructure and how devices are added to the central system.
* What LAVA does.
* References for more details on LAVA and testing jobs.

This closes OSTC/planning/testing#50

Signed-off-by: Gururaj Shetty <gururaj.shetty@huawei.com>