[📑 Docs]: document adding messages

[quetzalliwrites]

What Dev Docs changes are you proposing?

As part of Google Season of Docs 2023 at AsyncAPI, we're going to write in-depth explanations of the different sections of an AsyncAPI document to avoid difficulties in implementing EDAs.

This task is part of the Docs EPIC: #1507.

For this issue, we'll cover:

• Adding messages - Add a simple message as in previous examples, explain the most important parts, and explain there can be one or multiple (oneOf) under operation. Talk about contentType and add a note about defaultSchema. Also, show how to reuse components.

Code of Conduct

• I agree to follow this project's Code of Conduct

[smoya]

I suggest we clarify in the description of this issue that this doc should be based on AsyncAPI Spec version 3.

[mhmohona]

working on it.