Add guide for documentation-9-"RNN模型"

[shanyi15]

• Task
  Here is the reclassified documentation structure, please add a guide for "RNN模型", introducing what is "RNN模型" , and the general idea/function/target reader of each part.

Please note that:
现在的名字是“RNN相关模型”，将于2月9日改成“RNN模型”，请按照“RNN模型”来写guide,感谢！

• What is guide?
  It is an introduction of each part of one module, also, a brief introduction of this module is necessary. Take TensorFlow for example:
  

• Note
  Language:
  Please write in Chinese.
  Timeline:
  Draft: 1st Mar
  Review: 8th Mar
  Deadline:15th Mar
  Documentation structure mind map:
  mind map_cn
  mind map_en

请注意
直接改类似 doc/howto/capi/index_cn.rst的rst文件,不是新加一个md文件

• Background

About guide, we have discussed in our MRD, see page 6

Our goal is to make it clear and understandable for developers to use.

English version is also necessary, however, our plan is writing Chinese version this time.

Thank you so much!

---

Project Background
Optimize documentation of PaddlePaddle, enhancing user experience
-----Enhancement Plan: Stage 1 MRD
-----Enhancement Schedule（Update everyday）
-----Guide-adding Plan

[shanyi15]

@guoshengCS Please note that:
现在的名字是“RNN相关模型”，将于2月9日改成“RNN模型”，请按照“RNN模型”来写guide,感谢！

[shanyi15]

#8278 改名字的pr在这里

[guoshengCS]

@shanyi15 好的，收到