Add list of contributors to the homepage

[daanboer]

The list can be similar to the equivalent on LXCat 2 (https://nl.lxcat.net/contributors/), but condensed into a table. The headers of this table can be: contributor name, name of contact, contact email, link to LXCat 2 database, and perhaps a count of how many datasets are currently available LXCat 3.