Extend Jekyll build to use layouts, to ease the addition of new experiments.

[ric-bianchi]

Currently, atlas.html and cms.html are almost identical.

At the moment, we use Jekyll to build the list of images for the experiments and to dynamically create the listing.html file, which is used by components/load-assets.js to load the images.

We could further push the use of its functionalities. We could create a layout called experiment.html, to simplify the current site (atlas.html and cms.html could use the same layout, only changing the very few relevant parts) and to ease the addition of new experiments in a longer term.