Improve “Payment Gateway Types”

[judus]

I don't have a suggestion for https://craftcms.com/docs/commerce/4.x/extend/payment-gateway-types.html

• Add workflow charts
• Add code examples
• Do actually explain what a Gateway Type is - you only say that we can add our own and reference the Dummy Gateway
• Explain when, where and why hashes/reference and other numbers get generated
• Make it clear which components (controllers, models, services etc..) are in play and which ones are interchangeable (all of course, but which ones make sense to alter and when)

[gaelpleeroy]

I second this, I would love a more in-depth explanation of the Gateways.

[AugustMiller]

Still doing a bit of learning on Gateways, myself—but as a means to make this clearer, I created this PR that adds a few generators to Commerce.

That won't be the complete picture, but figured I'd at least signal that this is something we're thinking about!