Document Antora pipeline extensions

Antora 3 introduces an event-based extension mechanism to hook into the pipeline steps performed by the default site generator (and any generator that honors the same contract). Document this extension mechanism so an Antora user can make use of it.

The documentation should explain how the extension mechanism works, list the available events, explain how to create an extension that listens and responds to events, explain how to register the extension, and cover the pipeline APIs and context variables that are available to extensions.