Introductory texts on overview pages

[han-f]

In #853 and #801 we identified the need to have some introductory text also on the factsheet pages themselves, I will draft text here that can be implemented once agreed.

cc @stap-m , @l-emele

Page list:

• [ ] Factsheets Overview #801 #853
• [ ] MFS #853
• [x] FFS #853
• [ ] SFS #853
• [ ] Schemas Page #746 #846 #850
• [x] Tutorials
• [x] 404 #385

[Ludee]

Thanks for pointing out this very important issue! I scanned the older issues and there are more pages that need an introduction text. I will extend this issue and set links.