add more example configurations for PaddlePaddle.

[lcy-seso]

As discussed, it is necessary to add more example configurations for PaddlePaddle, so that users can better know about how to use PaddlePaddle layers through examples. There may be two ways to do this, I hope to collection ideas about which one is better.

1. Do not remove the current demos directory, and add new example to it.
2. Create a new repo (like Tensorflow https://github.com/tensorflow/models).

Either way, each example has its own directory, we can put configuration file and doc in it.

[luotao1]

可以同时考虑 #1725 "remove old api demo"

[lcy-seso]

It is expected that these example configurations are more lightweight than PaddleBook, but more heavyweight than API doc.

[typhoonzero]

Agree with No. 2. It may be confusing when "demo" and PaddlePaddle book hold some same examples.

[qingqing01]

demo: 可提供完整演示，可以比book轻量，不用很多背景+模型介绍等，可以删掉和book重复的篇章吧。
model_config/example: 可以只提供配置，不用完整演示吧，和demo分开两个目录，或者另一个repo都可以。

[lcy-seso]

I support to create a new repo to reduce the maintenance cost.

[wangkuiyi]

Created new repo https://github.com/PaddlePaddle/models