* add paddle new save load doc

* move static save apis & polish multiple details

* add api mapping

* move apis place

* fix example code error

* polish model save load guide

* polish example code

* fix details example error

* polish guide doc

* change title format

* remove history reason

* remove other history reason

* add enable_static for old save/load

* add enable_static for old api

* modify nn.data into static.data

* Update index_cn.rst

* nn.data to static.data

* remove alias

* update dygraph doc; test=develop

* change dygraph to imperative; test=develop

* move impertive to beginer guid; test=develop