Links to demos and tutorials from the CSIS front page

[DenoBeno]

I have added a block linking to some of the CLARITY demos and tutorials on the CSIS front page. This is how it looks now:

This is realised as a custom block https://csis.myclimateservice.eu/block/7?destination=/admin/structure/block/block-content that is placed on "front content"

@patrickkaleta , @fgeyer16: I have implemented this directly in CSIS because of the upcoming demo. Please find a nicer way to present this and assure that we don't lose this block on next update.