Reword welcome layout

[jum-s]

I would like to update what is currently in the third box of the welcome layout, as there are more features now than when this exchange part was written in 2015, which makes it quite secondary compared to other features from old ones (groups, data editor) to new ones (listings).

So, I would propose something about the bibliographical data.

It could be something like Contribute to an open catalogue with a Wikidata mention, or more on the book information one can get through inventaire: Find ideas for the next book you will read, Browse users inventories and book lists.

Also the second box users could mention the term group : Make friends, create groups and exchange books together (which would then wrap up the exchange part).