Documentation: KUnit: Added KUnit Architecture

Describe the components of KUnit and how the kernel mode parts
interact with kunit_tool.

Signed-off-by: Harinder Singh <sharinder@google.com>
Co-developed-by: Marco Elver <elver@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Link: https://lore.kernel.org/r/20211217044911.798817-4-sharinder@google.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>