I guess I have to interprete it literally that heroes can place a door ANYWHERE on the map.

So they are not restricted to place the door between walls? They could even place it through a wall if it would benefit them somehow?

See the door placement rules from the Bespin Gambit rulebook, page 4.

A door must be placed on exactly two edges (i.e. aligned properly). A door cannot overlap (or intersect) another door or a wall.

Overlooked those rules.

But the door does not have to be between two walls, does it? The rebels could place it in a 90 degrees angle to the stairs to block the way, couldn't they?

A placed door does not need to be between walls. It can be anywhere the rebels want as long as it does not overlap another door or wall (and is exactly covering two edges of spaces).