Change menu "Language Guide" into "News"

[wangkuiyi]

I feel that we can rename Document into Guide, and make it as when the user clicks Guide, it goes to a leaf of the documentation tree named "Guide".

The goal is the leave the space of "Language Guide" to "Blog" or "News"

[typhoonzero]

Agree!

[wangkuiyi]

If we can fix #85, we could also rename "ElasticDL" into "Playground".