As cookie-cutter mansions spread across the East End, the Architectural Sessions, a series of panels and presentations organized by the Parrish Art Museum in Water Mill and A.I.A. Peconic, continues to provide fascinating perspectives on architecture’s many possibilities. Friday’s program, “Pro Bono: Architects Who Serve Humanity,” focused on two unique projects, one in Rwanda, the other in Brooklyn, that are, in different ways, about creating something of value for their communities.