Chris Willman Senior Music Writer and Chief Music Critic If Memorial Day weekend is the unofficial start of summer on most Americans’ calendars, then it makes sense that the real tentpole kicking off tent season for a lot of people is not “Furiosa” or “Garfield” but “The Beach Boys,” a streaming documentary devoted to the least wintery group of all time. The Disney+ film, co-directed by music-doc stalwarts Frank Marshall and Thom Zimny, focuses on the first decade and a half, in the 1960s and ’70s, of the career of the rock band that still might rightfully be considered America’s greatest all-time group.