Boston Globe: New York Times Co. chief executive Janet L. Robinson, who led the company through some of its most challenging years and helped it become a multimedia force, will retire at the end of the month, the Times Co. announced yesterday. The company, which owns The Boston Globe, is beginning a search for her successor and said it will interview candidates internally and externally. Arthur Sulzberger Jr., chairman of the company and publisher of The New York Times, will serve as chief executive on an interim basis.