(#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.
Reminder to take this out before landing without the plugins page