Payment chapter, should have a section with listed available payment apps

[korycins]

The payment chapter describes the flow for processing the payment. https://docs.saleor.io/docs/3.x/developer/payments#introduction We don't mention there what kind of payment apps already exist. We have a section: https://docs.saleor.io/docs/3.x/developer/app-store/apps/overview where Stripe and Adyen are described. The payment section should have a ref link to the list of all payment apps.