V2 docs (#1830)

* Added v2 docs

* Move docs around

* Update _sidebar.md

* Update README.md

* Delete

* Remove usage docs (it was duplicated)

* Fixed sidebar links

docs/_sidebar.md

 - Getting started
- - [Installation - iOS](/installation-ios)
- - [Installation - Android](/installation-android)
- - [Usage](/usage)
+ - [Installation - iOS](/docs/installation-ios)
+ - [Installation - Android](/docs/installation-android)
+ - [Usage](/docs/usage)