Documentation: KUnit: update links in the index page

Replace out-of-date external links with references to the kernel
documentation, replacing TAP webpage for the more appropriate KTAP
documentation and the UML webpage by its documentation.

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>