Add documentation introducing payment service providers

[benjaminwil]

This adds an article about payment service providers. If you intend to accept payments in Solidus, you need to integrate with a third-party payment service provider—and there are a lot of ways to do that, including some existing Solidus extensions.

In the future, I'd like to supplement this article with some kind of tutorial. For example, a tutorial about integrating the Stripe provider with your own Solidus store. For now, this article just outlines some basic information about where to start with payment service providers.

This is part a larger project to improve Solidus's documentation. See this gist with the high-level table of contents. Where and how this documentation will exist is still up for discussion.