Create documentation concept for controller usage and hooks

[schleichardt]

Given I want to use a controller, like the ProductDetailPageController:

• [ ] how do I wire it into my application? (routes, DI)
• [ ] how do I add extensions at different points
  • [ ] for example, fetch a custom object requiring only the request parameter
  • [ ] fetch suggested products based on the fetched product (currently it is wired into the controller, but it should be like an add-on)
  • [ ] add data to the templates
  • [ ] support different overrides, I can use the Controller as base class for two different kinds of PDPs
• [ ] do we document as text which method is called when or do we some more magical thing to create the docu like a https://en.wikipedia.org/wiki/Nassi%E2%80%93Shneiderman_diagram?
• [ ] how do we create an overview about controllers?