Added solution that uses Occurrent

[johanhaleby]

This is an initial PR that adds examples for Occurrent. I haven't spent much time with the docs since I'm not sure whether you like additional examples or not :) I've also used some high-level components from Occurrent that does some of the work on behalf of the user (such as the use of a CloudEventConverter for Jackson to transform domain events to CloudEvents automatically and an application service for writing domain events to the ES). Not sure if this is counter the purpose of the exercise, if so I can change the implementation to be closer to the ESDB examples. Feel free to simply ignore the PR if you don't like it.