Improve documentation regarding pub/sub raw messages

**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](https://docs.dapr.io/developing-applications/building-blocks/pubsub/pubsub-cloudevents/#publishing-raw-messages) 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**
https://github.com/dapr/dotnet-sdk/issues/1294