[Docs] Add model docs (#416)

* first commit

* tmp commit

* tmp commit

* tmp

* finish base module

* tmp commit

* tmp commit

* first finish

* minor refine

* clean the code

* delete design

* refine

* minor refine

* fix as comment

* fix as comment

* refine from jupyter notebook

* minor refine

* split model.md to model.md and initialize.md

* minor refine

* minor refine

* make link availiable

* fix as chengqi comment

* fix as chengqi comment

* fix link

* fix as wenwei comment

* Fix as comment

* Fix as comment

* Fix lint