Missing documentation for notification events

[Slawek Kaplonski]

Since very long time Neutron have pecan hook which can send notifications about various events start and end: https://github.com/openstack/neutron/blob/master/neutron/pecan_wsgi/hooks/notifier.py
We do have some documentation about how to configure those notifications: https://docs.openstack.org/neutron/latest/admin/archives/adv-operational-features.html but we don't have any documentation about events and their payload.
For example nova have something like that in: https://docs.openstack.org/nova/latest/reference/notifications.html

We should add something similar in our documentation.