[Docs] Refine config docs. (#432)

* tmp commit

* refine docstring

* minor refine

* fix as comment

* add tutorial for merge_from_dict

* minor refine

* minor refine

* export md by jupyter

* temp add ipnb

* refine the docs

* remote ipynb

* fix as comment

* add change _base_.variable

* Fix as comment

* minor refine

* Fix as comment

* Fix as comment

* minor refine