* 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)

* migrate en doc from old dir

* fix typo

* fix typo

* fix typo

* add input_spec cn

* add input_spec cn

* fix === style

* fix file name typo

* add ref

* add en doc

* fix en doc

* refine according reviewer

* remove user comment

* refine doc

* refine doc

* add migration test=develop

* fix_logs test=develop

* fix_migration_guide and add dygraph to static test=develop

* fix_install test=develop

* fix install and args

* fix_style test=develop

* fix docs test=develop