Add option to show carousel images as gallery

[lassoan]

We expect to collect many dozens of example images to accumulate over time. Users would not be able to explore these many images with a carousel. It would be important to also have a gallery view of the same image list.

[jcfr]

From @lassoan

There is no strict limit on the number of items, but probably readers would not scroll through more than 20-30 items. So these items should be strategically chosen to give a balanced, strategic overview of what Slicer can be used for. Something like a few items of each major feature, a few items for each major extension, etc