Improve documentation regarding pub/sub raw messages

[fabistb]

What content needs to be created or modified?
The dapr pub/sub raw documentation provides a good overview about this functionality.
However the details can be hard to understand.

Especially when just briefly checked the introduction sentences you can get the impression that the CloudEvent is just removed and otherwise it works like before.

Dapr enables apps to publish and subscribe to raw events not wrapped in a CloudEvent for compatibility.

Describe the solution you'd like
Add a more detailed documentation how the pub/sub raw functionality works especially taking the different components into consideration.

Where should the new material be placed?
https://docs.dapr.io/developing-applications/building-blocks/pubsub/pubsub-raw/

Additional context
dapr/dotnet-sdk#1294

[msfussell]

@fabistb - Can you add more details on expectations here, what is it that you ran into that should be covered better. Could you be more specific?