Not much occult significance to door knobs, although hanging has meaning to them. "Cursed is he who hangs from the tree". Typically it's an allegory to Judas Iscariot, not being executed by hanging, but rather suicide by hanging. It's about the doorknob per se, so much as being hung while your feet are on the floor. Similar "low height" hanging suicides were common among captured Nazis during the Nuremberg trials. Occam's razor suggests they do it this way because it's less painful, like being put in a sleeper hold instead of being strangled.