Enhance topics documentation for Kafka eventbus

[Hronom]

Is your feature request related to a problem? Please describe.
Currently from description here https://argoproj.github.io/argo-events/eventbus/kafka/#topics it's hard to understand on why each topic is needed and how Argo Events will use it.

Currently EventBus based on Kafka looks like a black box. And in order to build robust system we need more context.

Describe the solution you'd like
Need more info from maintainers about:

1. How each topic event, trigger, action is used by Argo Events
2. If we have messages with different payload, should we create several EventBus or one EventBus is enough
3. Recommendations about what partitions we need to set for topics if we expect high amount of messages. Should we set number of partitions for event, trigger, action topics or only for events.

Also whatever you think will be useful to understand on how we can work with it.

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