Add some additional documentation to the page

[foosel]

Problem

There are now so many settings and possible combinations of them that it gets a bit confusing when you don't have happened to written the thing yourself.

Solution ideas

Add some included documentation, e.g. in modals that can be opened by clicking on a "?" or something, that explain

• the various card back options
• the various margins in grid settings and PDF generation
• the basic ideas behind the algorithm that decides how to orient the cards on the foldable page
• vertical vs horizontal fold lines and when to use what
• ...