example: Put the content in a scrolled window

This ensures the example can fit windows of any height.

This also makes the stack containing the content non vertically
homogeneous so the scrollbar appears only on examples needing it, while
keeping it horizontally homogeneous for to keep when the leaflets will
be folded consistent.
4 jobs for example-scrolled-window in 2 minutes and 54 seconds (queued for 1 second)