[📑 Docs]: document securing operations

[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:

• Securing operations - Explain that server security applies to all the operations in all channels. Still, if you have different security per operation, you can use the security prop at the operation level.

Code of Conduct

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

[quetzalliwrites]

assigned @BhaswatiRoy ✨

[smoya]

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