[Templates] Add template "call-for-model" markdown and "call-for-big-bird" markdown (#9921)

* add big bird

* change teacher to mentor

* add proposal template

* adapt template

* delete old template

* correct some links

* finish template

* create big bird from template

* add big bird

* improve boxes

* finish boxes

* add pointers for BigBird

* finish big bird

* up

* up

* up

* up

* apply lysandres and sylvains suggestions

* delete bogus file

* correct markdown

* try different style

* try different style

* finalize

templates/adding_a_new_model/open_model_proposals/README.md

 Currently the following model proposals are available:
 
-- TODO(PVP) add BigBird
+- [BigBird (Google)](./ADD_BIG_BIRD.md)