Document listeners

Jetty now has many listener APIs, both standard and proprietary. It can be a challenge for a developer to know about all the listeners that are available, so it would be good to develop a comprehensive section of the documentation (and/or) blog that enumerators all our listeners and their methods. A very partial list includes:
- [LifeCycle.Listener](https://github.com/eclipse/jetty.project/blob/jetty-9.4.x/jetty-util/src/main/java/org/eclipse/jetty/util/component/LifeCycle.java#L117)
- [Connection.Listener](https://github.com/eclipse/jetty.project/blob/jetty-9.4.x/jetty-io/src/main/java/org/eclipse/jetty/io/Connection.java#L129)
- [HttpChannel.Listener](https://github.com/eclipse/jetty.project/blob/jetty-9.4.x/jetty-server/src/main/java/org/eclipse/jetty/server/HttpChannel.java#L1031)
- All the standard servlet listeners
- etc.

It would be good if we could enumerate, together with:
- what object are they set on
- how are they inherited
- exactly when they are called (names can be deceiving, specially for the standard listeners)
- what is their calling scope? Do they have a webapp context classloader?
- can the impl block?