Add basic overview of the event system

[mliszcz]

The documentation has a page dedicated to the event system:
https://tango-controls.readthedocs.io/en/latest/administration/services/events.html
However it describes only multicast setup which is somewhat advanced topic without wide adoption.

As proposed at the Tango Kernel teleconf meeting held on 2020-02-27, we should update this page and give a basic overview of the event system: what works out-of-the box, what can be configured, etc. ...

Action - S2Innovation?: Create an issue in tango-doc (if does not exist yet) to ask for an improvement of the event system documentation.

https://github.com/tango-controls/tango-kernel-followup/blob/master/2020/2020-02-27/Minutes.md#summary-of-remaining-actions