Merge pull request iissnan#1712 from Jacksgong/fix-mathjax-cdn

fix: using bootcss cdn instead of mathjax official one because the official cdn has been closed

Author: ivan-nginx

_config.yml

@@ -293,8 +293,7 @@ highlight_theme: normal
 mathjax:
   enable: false
   per_page: false
-  cdn: //cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML
-
+  cdn: //cdn.bootcss.com/mathjax/2.7.1/latest.js?config=TeX-AMS-MML_HTMLorMML
 
 # Han Support docs: https://hanzi.pro/
 han: false