Add updated <Provider /> docs

[maecapozzi]

Purpose

Related to #1001

We're working on rewriting the react-redux documentation completely. This is the first PR opened to begin that process.

The goal of these changes is to make the section on the <Provider /> more readable and approachable for react-redux users. I'm looking for suggestions on grammar, any missing information, and a sanity check on my code examples.

Approach

Folder Structure
I have added a new folder structure based on @markerikson's suggestions. Documentation will continue to live under /docs, but I've added a folder at /docs/api and a Provider.md file.

Updated Docs
I've reworked some of what currently lives in api.md as well as referenced some popular tutorials to try to incorporate some language that might be familiar to current users of react-redux.

[markerikson]

First suggestion: actually save the file with the contents inside and THEN push :)

[markerikson]

Maybe doesn't need to be in there right now, but I sometimes see questions from people unsure how to do this in React Native. Part of that is that navigation in React Native varies based on what lib you're using, and there's also not an obvious call like ReactDOM.render(). Having an example or two for React Native (maybe for a couple different navigation libs?) would be beneficial.

[markerikson]

Other than that, looks good. Same basic content we already had, but split out and cleaned up a bit. Aaaaand we're on the way!

[maecapozzi]

One more thing @markerikson forgot to mention before merging: <Provider /> throws an error if you give it more than one child. We might want to add this to the docs.