Documentation: kunit: add tips.rst for small examples

./usage.rst contains fairly long examples and explanations of things
like how to fake a class and how to use parameterized tests (and how you
could do table-driven tests yourself).

It's not exactly necessary information, so we add a new page with more
digestible tips like "use kunit_kzalloc() instead of kzalloc() so you
don't have to worry about calling kfree() yourself" and the like.

Change start.rst to point users to this new page first and let them know
that usage.rst is more of optional further reading.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>