[Docs] Add Guide to Write C++ TM (facebook#4254)

* [WIP]Add guide for Pure Cxx TM

* Integrate C++ code with iOS

* Remove video if too big

* Readd  video

* Add base info for Android

* Add Android integration

* fix sidebar

* Fix linter

* Fix case police

* Apply first round of feedbacks

* Apply other review feedback

Author: cipolleschi