Update docs: Add next option custom fetch options

[christophby]

In the Nextjs playground there's an example of using the next object as a custom fetch option.

 const res = await storyblokApi.get(
    `cdn/stories/home`,
    { version: 'draft' },
    {
      // cache: 'no-store',
      next: {
        revalidate: 3600,
      },
    }
  )

This should be added to the docs.

[alvarosabu]

Thanks for raising this @christophby I just shared it with the docs team.