Will Ferrell takes a ride across America with his best friend in “Will & Harper,” a sweet, kindhearted documentary about truth, discovery and the open road of friendship. Harper Steele is his co-pilot for the adventure, and she’s on her own path: The former “Saturday Night Live” writer underwent a gender transition during the Covid-19 pandemic, announcing her intent to Ferrell and a few close friends and family members by email. Harper had many questions herself – “will I still be loved?” and “where do we go from here?” among them – and she knew Ferrell would have his own questions about her t…