Missing info in Factsheet overview pages

[christian-rli]

Users found that documentation on factsheets was missing. Why are they there, how are they created and what's their relation to the data. Some, if not most of that information is available in the overview page. However people don't realize that's the place to find that information. It could be renamed to "About Factsheets" or something similar.

Also it would be helpful to have a short description on top of each the Model-, Scenario-, Framework- and Study Factsheets. A sentence or two could be enough and a link to the overview page.

Describe how a connection between Factsheets and the data tables is realized. This is connected to bundles.

[han-f]

Also related to https://github.com/OpenEnergyPlatform/tutorial/issues/147