Fix publiccode.yml file for publication

This PR:

• shortens the length of a feature description
• adds the logo locally, as mandated by the standard (see https://docs.italia.it/italia/developers-italia/publiccodeyml-en/en/master/schema.core.html?highlight=logo#key-logo)
• adds the screenshots locally, as mandated by the standard (see https://docs.italia.it/italia/developers-italia/publiccodeyml-en/en/master/schema.core.html?highlight=screenshot#section-description)

Feel free to squash it as you wish (or, if you want, I can do that). Thanks a lot and let me know if you have further questions. @lorello