Please adjust the structure of documentation

[shanyi15]

In order to enhance V2 website and docs structure, we have adjusted the structure of docs.

This is a Chinese Version mind map, English Version please see #8230

Please refer to the structure in this mind map and adjust 【ducumentation】（使用文档）,including: structure and name.
Note: this issue mainly focus on 【documentation】, Others，Like【API】，have been written in other issues.

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Project Background
Polish documentation of PaddlePaddle, enhancing user experience
-----Enhancement Plan:
Stage 1: https://note.youdao.com/share/?token=83F0AB634A7749E6B4676F823D6EC343&gid=69922175#/
-----Enhancement Schedule:
https://shimo.im/sheet/VnyT2IKRHjoU1a6i/6HGyl//

Thank you very much!

[luotao1]

We could remove mobile section in the left navigation bar, since we have the same context in the top navigation bar.