Is Itachi Shinden a separate anime series or part of Naruto Shippuden itself?
Itachi Shinden is in fact a part of the Naruto series. The myth is that Itachi Shinden will have its own anime, this is not true. Itachi Shinden will be an arc attached to the current Naruto Anime. In the current anime of Naruto, Itachi's life has been shown while he was active in the ANBU, however, the new arc introduces a new look to Itachi's life, his personal life. Not much is yet known but it's definitely an arc of Naruto seeing as it was also created by Masashi Kishimoto. 

Anyway, I hope this helped you find your answer.