Add cover image of New Jersey to the OCaml Workshop 2020

[ILeandersson]

Add this picture:

to the OCaml Workshop 2020 page so that it looks like this:

[patricoferris]

Yep sounds good thanks @ILeandersson, this might happen soon or it might wait on https://github.com/ocaml/ood/issues/58 (as we decide how to handle all the media). Separately, isn't this a picture of New York and not New Jersey ? Perhaps we should use something like the "Jersey City from Manhatten" image from the ICFP site https://icfp20.sigplan.org/ :))