Bug fixes and improvements (#1205)

* Add Ruby code blocks to documents

* Remove Ruby code from en/docs

* Remove "center-table" class in index.md

* Add "data-toc-label" to handle the latex heading during the build process

* Use normal JD link instead.

* Bug fixes

docs/chapter_backtracking/index.md

@@ -1,11 +1,7 @@
 # 回溯
 
-<div class="center-table" markdown>
-
 ![回溯](../assets/covers/chapter_backtracking.jpg)
 
-</div>
-
 !!! abstract
 
     我们如同迷宫中的探索者，在前进的道路上可能会遇到困难。