Add information about possibility to import collections

Hello @bbrancot

Taking into consideration the reviewers' comments, and after discussion with the other authors, we would like to add information about possibility to import collections and in this case also set up specific identifiers.

The idea is to modify the site as well as the documentatation in order to let the user know that is it possible to import big collections and set up the corresponding identifiers. For this, the users should contact us directly with the contact email already existing.

The corresponding collection and identifiers would be implemented similarely to how the Felix d'Herelle collection was implemented.