Add description for aggregation page

[mandrasch]

https://oerworldmap.org/aggregation/

My guess: User does not know, that he/she can click on the statistic elements.

Use explanation/help text like in OER policy registry?

Also there is the need of an explanation what the global statistics are.

Also the closing button on the end is not very helpful if user enters the page via link directly (we have a link to that on the landing page)