Having joked to “Today” show host Matt Lauer yesterday that the poor box office returns on “Bicentennial Man” have forced him into this situation, Robin Williams is now on the Net. On Wednesday the comedian and Oscar-winner (for “Good Will Hunting”) unveiled his own Internet audio site, RobinWilliams@audible.com, a hodgepodge of comedy bits old and new, delivered only as Williams can. These include him mimicking Jimmy Stewart reading a Jackie Collins novel and Jack Nicholson reading a children’s book. Additional material will be presented every week, Williams says. Soon the site will also provide the capability for users to e-mail Robin and Williams’s conversations with fellow comedians, including, he quipped on New York’s WNBC TV this morning, the Pope.