markdown语法：
[to_be_link](#id_name)

标题：
## 2.aaa <a name="id_name"></a>

参考：

http://stackoverflow.com/questions/3292903/in-markdown-what-is-the-best-way-to-link-to-a-fragment-of-a-page-i-e-some-id

gitlab markdown支持页面内跳转