Aiming to make Passover more accessible to the young adults, JewishBoston.com, an online hub for the area’s Jewish community, is offering its first do-it-yourself Seder kit. Free to Boston-area residents, ages 18 to 40, who sign up by April 8, “Seder in a Box’’ includes just about everything but the food and the guests: a Seder plate, a basic Haggadah, a leader’s guide, recipes, a shopping list, instructions for setting the table, and a matza cover.