Documentation: kunit: adds complete documentation for KUnit - Added intro and usage guide for KUnit - Added API reference Change-Id: I9f0969f328a2a812677b2c5c4a3c69a049bbb551 Signed-off-by: Felix Guo <felixguoxiuping@gmail.com> Signed-off-by: Brendan Higgins <brendanhiggins@google.com>