docs: Document the webhooks system

[cdecker]

Now that the outgoing node-webhooks are implemented we should also
document them, so developers can start building on top of them.

• Write an example in the Tutorials section on how to use webhooks
• Create an overview over the various webhook mechanisms we intend to offer:
  • outgoing node webhooks
  • outgoing app webhooks (through the GDC, allows developers to register or deregister an app-wide webhook called for any node from its installation base)
  • incoming webhooks (LSPs and others telling us we should wake up