* 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

We think we should split the usage and architecture because of clear organization and future extension (we may add more usage guide)