Create a Marketing Working group

[lttucker23]

We propose creating a marketing working group for the AGNTCY project. (see discussion https://github.com/orgs/agntcy/discussions/40)

Proposed charter: https://docs.google.com/document/d/17c02Ee6MlsRqRcZWasDqWFhojc4F-acrZHbzgxCjmqk

cc @tiswanso @natalisucks