Add gatewayd plugin scaffold CLI docs

[mostafa]

Ticket(s)

• plugin scaffold: Create a plugin skeleton/scaffold (init) #475

Description

This PR update CLI docs.

FYI: @zeina1i

Related PRs

• Implement plugin scaffold #518

Development Checklist

• I have added a descriptive title to this PR.
• I have squashed related commits together.
• I have rebased my branch on top of the latest main branch.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added docstring(s) to my code.
• I have made corresponding changes to the documentation (docs).
• I have updated docs using make gen-docs command.
• I have added tests for my changes.
• I have signed all the commits.

Legal Checklist

• I have read the CONTRIBUTING document.
• I have read and understood the Code of Conduct.
• I have read and agreed to the Apache CLA (required).
• I have read and agreed to the LICENSE (required).

[zeina1i]

Thanks @mostafa for updating the docs
Your PR looks good to me 👍