FEATURE: do not need thumbnail and leverage runtime generated image

Currently, we propose our examples with a thumbnail and it is leveraged when generating the documentation.
This adds the constraint that each example is associated to a thumbnail stored in the repo.

It would be nice to get rid of this constraint and allow the documentation build to retrieve the first/last/somethingelse generated image as thumbnail. On top of easing the process of adding new examples it would lessen the weight of the repo !

Start by tracing the documentation build and how it currently uses repository thumbnails for examples. Define completion as allowing the build to select a generated image for each example while removing the requirement to store a separate thumbnail, then verify the documentation still builds correctly.

Written by the indexing model from the issue text.