Documentation: KUnit: add intro to the getting-started page

Describe the objective of the Getting Started page, which should be a
brief and beginner-friendly walkthrough for running and writing tests,
showing the reader where to find detailed instructions in other pages.

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by: Sadiya <Kazi&lt;Sadiaykazi@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>