Improve docs for when and how to add events

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.

Open docs.openedx.org/projects/openedx-events/en/latest/how-tos/creating-new-events.html and read the guidance on creating new events. Clarify that events serving a single use case are acceptable and common, while future consumers can still inform event design. Done means the page states both points clearly.