Jenny Popplewell’s true-crime documentary “American Murder: The Family Next Door” takes place in the kind of neighborhood where you can imagine somebody saying, “You don’t expect this sort of thing to happen here.” The corner of Frederick, Colorado we see in the movie is one of those sparkling new suburbs where the lawns and paint jobs are CGI bright and nothing much ever seems to happen, until it does. One can imagine a documentary in which the area’s very anonymity and a sudden, seemingly pointless murder in its midst are tied into a larger narrative. That is not this movie.