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>
8 files changed