[RN][Docs] Add guide on custom types in C++ (facebook#4269)

* [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

* Add guide on custom types in C++

* fix broken link

* Apply review feedback

* Fix note block

Author: cipolleschi