Documentation: kunit: Fix "How Do I Use This" / "Next Steps" sections

The "How Do I Use This" section of index.rst and "Next Steps" section of
start.rst were just copies of the table of contents, and therefore
weren't really useful either when looking a sphinx generated output
(which already had the TOC visible) or when reading the source (where
it's just a list of files that ls could give you).

Instead, provide a small number of concrete next steps, and a bit more
description about what the pages contain.

This also removes the broken reference to 'tips.rst', which was
previously removed.

Fixed git am whitespace complaints during commit:
Shuah Khan <skhan@linuxfoundation.org>

Fixes: 4399c737a97d ("Documentation: kunit: Remove redundant 'tips.rst' page")
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Sadiya Kazi <sadiyakazi@google.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>