[Docs] Add docs coverage for Vanilla JS

[nimonian]

The docs introduce us to Vanilla, React and React Native

It isn't clear that the remainder of the docs are only relevant to React (although we infer it pretty quick, it's a tad confusing) - I've been spelunking the codebase to figure out how to use the Vanilla SDK

It would be good to have a dropdown or perhaps tabbed code samples which allow us to switch between SDKs, and allow for full coverage of the Vanilla SDK