docs: publish plugins from CI with API keys

[bajrangCoder]

• New "Publish from CI" guide: creating an API key, publishing with the Acode-Foundation/publish-plugin GitHub Action, and calling PUT/POST /api/plugin with curl
• Status codes, what keys can't do, and tips for keeping keys safe
• Links to the guide from "Create new plugin" and the manifest's "Publishing updates"
• Sidebar entry under Getting Started

[greptile-apps]

Confidence Score: 5/5

[Low impact] The PR appears safe to merge; the previous finding is fixed and no new issues were found.

Summary

Adds a guide for publishing plugins from CI.

• Plugin authors now have a guide for publishing releases from CI.

_{Reviews (2) · Last reviewed commit: "docs: note price changes need the websit..." · Reviewed by Greptile}