Documentation: KUnit: Rewrite main page

Add a section on advantages of unit testing, how to write unit tests,
KUnit features and Prerequisites.

Signed-off-by: Harinder Singh <sharinder@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Link: https://lore.kernel.org/r/20211217044911.798817-2-sharinder@google.com


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