docs: [FC-0074] explain how the event bus works and how to use it

[mariajgrimaldi]

Description

This PR adds sections dedicated to the Open edX Event bus technology:

1. An Overview of the technology:
   What is the Open edX Event Bus?
   Why use the Open edX Event Bus?
   How Does the Open edX Event Bus Work? With an architectural diagram.
   How is the Open edX Event Bus Used?

2. Using the Open edX Event Bus: install event-bus plugins, configure them, produce events, consume events, run the consumer

3. Event Bus Configurations: the available configurations taken from in-line code annotations.

4. Glossary terms for the event-bus: event bus, message, message broker, worker, producer, consumer, topic

These docs were written based on the available documentation about the Open edX Event Bus:
https://openedx.atlassian.net/wiki/spaces/AC/pages/3508699151/How+to+start+using+the+Event+Bus
https://openedx.atlassian.net/wiki/spaces/AC/pages/3474784257/Event+Bus+Architecture+Overview

This PR partially addresses #238

[openedx-webhooks]

Thanks for the pull request, @mariajgrimaldi!

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[mariajgrimaldi]

This was moved to this file: https://github.com/openedx/openedx-events/blob/3954196516163fa6f31561885e251e63c6c6e50b/docs/how-tos/using-the-event-bus.rst

[mariajgrimaldi]

Maybe we should add a redirect here?

[sarina]

Should these docs live here? Or on docs.openedx.org? (I don't know the right answer just asking)