Documentation improvements

[dbalabka]

At first glance, the Payum library looks very attractive in terms of the number of Payment Gateways it supports. However, after I read the documentation, I found that it does not cover well enught very important aspects that can partially be obtained by reading the library's source. I believe that newcomers face a very steep learning curve. It would be great to improve the documentation to improve developers' experience.

Here are specific areas that can equip others with the necessary understanding of the Payum library for the successful integration of new payment gateways:

1. Many core entities, such as Gateway, Action, Request, Storage, Context and many others, require more details about what they do and when we need them.
2. Better gateway understanding. While default gateway implementation is very straightforward, documentation is missing more details on how it executes the actions. Also, I found that it has a stack of actions that isn't mentioned in the documentation. Here is the primary method that explains the main flow: https://github.com/Payum/Payum/blob/master/src/Payum/Core/Gateway.php#L75
3. The documentation could benefit from more comprehensive explanations of the numerous default Requests and Actions. This would strengthen developers' support in their integration process. For example, I would like to understand what a minimal list of Actions I must implement to integrate some payment solution.
4. It would be great to read about the motivation for choosing such an approach to the architecture for Payment Gateways. Payum's approach looks a bit cumbersome with their Gateway, Action, Requests, Storages, etc.
5. Payment Gateway usually use a very similar approach to capture the payment and API integration. It would be great to summarise the experience of the author of this library and provide some receipts for implementing a new payment gateway integration. Currently, documentation provides overly simplistic examples and no connections to the real world. Digging into existing source code isn't helpful enough.

I searched tickets that were reported over time and found a few related: #907, #796, #774, #854, #792, #769, #651, #418