JuliaGP landing webpage

[theogf]

As discussed in the JuliaGP meeting, we need a better unification of the whole ecosystem which could happen through a general website

Right now we have https://juliagaussianprocesses.github.io/, which contains only a landing page, links to the different repos, and the team section.

A proposition is to add more content to describe the use of the different repos and to have a "gallery" of fancy "end products" (plots, animations, etc) linking directly to the related tutorial.