(#2766) - add the PouchDB guide

I tried to think of something meaningful to add this guide, and I couldn't. I think it's done.

In the future, we may want to add chapters on plugins, testing, and local documents, but for now I think it covers the basics, and that's way better than what we've had up 'till now.

Let's merge this, and give people something better than just the API docs.