[DOC] Update link in docs header to point to virtualship website

[VeckoTheGecko]

This is the header for the website at the moment

Where "OceanParcels" links to https://oceanparcels.org/. But there is no mention of the https://virtualship.oceanparcels.org/ website. Would it make sense to add a link to the website instead of oceanparcels? Or have both links?

[erikvansebille]

Yep, a link to virtualship.oceanparcels.org would make much more sense!