Merge branch 'markdown' into 'master'

Update index.yml

See merge request codechina/operation-work!24

explore/index.yml

 banner: 
 #  - url: 'https://codechina.csdn.net/codechina/operation-work/uploads/2e60ed02850be2a2e712e22c0221004e/banner-show.png'
 #    link: 'https://codechina.csdn.net/lives'
-  - url: 'https://codechina.csdn.net/codechina/operation-work/uploads/e48b8c8b72c35958468d0e69bec41f6b/1920x492.png'
-    link: 'https://codechina.csdn.net/courses?utm_source=explore'
+  - url: 'https://codechina.csdn.net/codechina/operation-work/uploads/6a42cb09830df2ead3d3d62cde6bc1b5/markdown-banner.jpg'
+    link: 'https://codechina.csdn.net/courses/detail/2/l?utm_source=explore'
 learning_banner: 
   - url: 'https://codechina.csdn.net/codechina/operation-work/uploads/38be577289ba8800a9caa233416cd4b6/1920x480_1_.png'
     link: 'https://codechina.csdn.net/courses'