Optimisation: only rebuild github pages when needed

[geoffroy-noel-ddh]

The publication is too slow because it does unnecessary transfers:

• files which are too large (images)
• files that have not changed
• files that are not needed by the front-end (?)
• non-important changes (e.g. evaluate.json shouldn't trigger publication)

203M projects (screenshots)

This may require calling resources via github (even when token unset), reorganising the files or creating a deployment script.

Look into github actions caching.