Improve docs for when and how to add events

[robrap]

This would likely affect:
https://docs.openedx.org/projects/openedx-events/en/latest/how-tos/creating-new-events.html

A quote from someone about something that wasn't obvious from the docs:

I think what I’m hearing is let’s not be stingy about creating new events even if it only has one use today

I think it would be great to clarify that events that only serve a single use case (to start) is not only completely acceptable, but the most common situation. That said, thinking about future consumers might help influence the event design, so it is useful to consider as well.

[robrap]

@Zacharis278: You mentioned "best practices", but I think it is difficult to start from something so general. If you have specifics about things you learned, or questions you have, I think it would be great to capture them on this ticket, and eventually in the docs. Thank you!