contributing/devtool usage: Introduce new topic devtool

* Added new topic devtool in contributing, fixes
  OSTC/OHOS/docs#72 and
  OSTC/planning/core-os#188

* Index.rst modifed to include the new devtool topic.

Signed-off-by: Arul Kumaran <arulkumaran.devarajan@huawei.com>