Several bug fixes (#984)

* Fix quick_sort.md

* Disable instant loading for mathjax rendering

* Fix the summary.md of chapter_sorting

* Two bug fixes

mkdocs.yml

@@ -30,7 +30,7 @@ theme:
     # - header.autohide
     # - navigation.expand
     - navigation.indexes
-    - navigation.instant
+    # - navigation.instant
     # - navigation.prune
     # - navigation.sections
     # - navigation.tabs