Documentation: kunit: Make the KUnit documentation less UML-specific

Remove some of the outmoded "Why KUnit" rationale, and move some
UML-specific information to the kunit_tool page. Also update the Getting
Started guide to mention running tests without the kunit_tool wrapper.

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>