create a hook system

[Boldewyn]

• add initial hook layout and one first hook
• log used customer-provided hooks to DB
• begin general and hook documentation
• test the hook infrastructure
• add two more filters to allow pre- and postprocessing of content
• semi-automatically document hooks

ERM48560

[Boldewyn]

Ready for review. Currently we only have 3 hooks. I’ll add some more useful hooks in a follow-up PR.