fix some junk from yesteryear in the contracts reference
break some lines in the ->i blue box
to make it fit in the allowed width
adjust the drracket blue boxes to be more like the new docs blue boxes
fix contract
Correct type of hash-has-key?.

Closes PR 14157.
integer is now natural, Closes PR14156
Simplify type of hash-has-key?.
fix contract (#f can always happen-- it means the dialog was cancelled by the user)
use the same colors as in the docs for the text & label in the blue boxes
Guard drag-ok? so it handles the empty list properly

change suggested by Stefan Schmiedl; closes PR 14164

It seems a bit strange that a region callback can
be invoked when there are no cards, but looking at
the card library it isn't obvious that the list
is supposed to be non-empty, so probably better
to just be "defensive" in this case, rather than
attempting to tighten the contracts
fix custom deserialize-info

The deserialize fallback fails on modules that contain both custom
deserialize-info definitions and uses of serializable-struct.
add check for check-exn predicate
 closes PR 14165