Document how to add {namespace} option to set consumerID for Pub/Sub namespace scoping

[greenie-msft]

What content needs to be created or modified?
Document how to add {namespace} option to set consumerID for Pub/Sub namespace scoping
These images would be valuable to add:

Describe the solution you'd like
Under the pub/sub building block, consider a new how-to for namespace scoping with pub/sub

The associated pull request from dapr/dapr, dapr/components-contrib, or other Dapr code repos

dapr/dapr#5112

[msfussell]

@hhunter-ms - Would you be able to take this issue. This video explains the features. https://www.youtube.com/watch?v=eK463jugo0c

This should be updated in the following topic

1. Add namespacing support as a new feature into overview here https://v1-9.docs.dapr.io/developing-applications/building-blocks/pubsub/pubsub-overview/#dapr-pubsub-api-features
2. Create a new HowTo topic (use a template!!) based on this video https://www.youtube.com/watch?v=eK463jugo0c as an example. Does not have to have code. However should include diagrams and links to this video
   In this article reference https://docs.dapr.io/operations/components/setup-pubsub/pubsub-namespaces/ as another topic to be aware of.

(only as FYI, you will see there is namespacing in Service Invocation also today. See https://v1-9.docs.dapr.io/developing-applications/building-blocks/service-invocation/service-invocation-namespaces/. At the same time can you fix up this article to say "How To" at the start to make it consistent.

[greenie-msft]

Closing since PR has been merged #2864