Are Zhangjiakou people descendants of Qidan people?

No, the Khitan is divided into two ruling modes, the Khitan nationality in the north and the Han nationality in the south. The Han nationality is distributed in sixteen states, including Beijing, Tianjin, northern Shanxi and Hebei, and Zhangjiakou is one of them. Many people in Zhangjiakou moved here from Shanxi during the Ming Dynasty. So Zhangjiakou people have nothing to do with Qidan.