theme: optimize page load for images

[McShelby]

Loading the print preview for the exampleSite shows some potential for improvement when loading img elements.

At best, we could give a size to the elements and load them async. This would require some Hugo render hooks.

[McShelby]

As of now, we now have a render-image hook which makes things a little easier to be started.

Once done, assert with Lighthouse that everything is fine.