(#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.
The highlighting of Buffers / Blobs is strange, not sure if we want to highlight the whole word or just ignore it