fix: cover overlapping sidebar by removing z-index
fix: cover overlapping sidebar by removing z-index

* Fix cover overlapping sidebar

* Fix transition of cover page

* Revmove transition property from .cover

* Remove z-index attribute from _coverpage.styl
nsina/docsify,QingWei-Li/docsify,nsina/docsify,nsina/docsify,QingWei-Li/docsify,QingWei-Li/docsify