Add a guide for creating a custom payment processor integration (OCC-251)

[sarahelsaig]

Is your feature request related to a problem? Please describe

I've encountered users interested in implementing integration for their own favorite payment providers. The topic is briefly covered in the docs, but it's still a harsh learning curve to get started.

Describe the solution you'd like

It would be good to add an article in the guides section of the docs, that gives a high level walkthrough of a payment provider implementation (e.g. OrchardCore.Commerce.Payment.Exactly), and discusses the best practices. Such a write-up would aid those who want to implement their own, take pressure off the core team and help build the OCC ecosystem.

Describe alternatives you've considered

We could also implement a code walkthrough (i.e. OrchardCore.Commerce.Payment.Samples), but I think that would be too much effort and would require too much mocking to be useful. In this case prose makes more sense.

Jira issue