Improved organization of event bus specific docs

[robrap]

I still think it is better to have the event bus work in this repo than to have the pain of maintaining yet another repo. However, it can be confusing for some what is the difference between events in general and the event bus.

I think it is good that we have the code separated out: https://github.com/openedx/openedx-events/tree/main/openedx_events/event_bus.

However, I'm wondering if we should have a separate directory for event bus specific docs as well? Some alternatives:

• [Option 1] Add docs/event_bus/how-tos, etc. (Simpler to add to TOC.)
• [Option 2] Add openedx_events/event_bus/docs/how-tos, etc. (Has some pain involved, but matches OEP-19, for now.)
• [Option 3] Leave as-is with purely event bus related docs intermingled with the rest.

Note that openedx/open-edx-proposals#530 provides more details about Option 1 and 2, especially in the linked edx-platform issue.