ENH: update the documentation link
ENH: update the documentation link

old gitbook is gone, the original URL https://qiicr.gitbooks.io/dcmqi-guide now redirects to https://qiicr.gitbook.io/dcmqi-guide/
QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi