Previously the docs order was defined by the default behavior of
Storybook: discovery order. But, in our situation we want the
"Getting Started" part to appear above "Components", and "Doc"
above any other stories. So we needed to create a custom sort callback.
In order to make this storySort callback to be taken into account by Storybook
it was needed to rename the file to preview.js. Strange but .. meh.
