Add Build with Codex documentation

[johnbhiggins7]

Summary

• Add a dedicated Build with Codex guide for Codex CLI and Codex in the ChatGPT desktop app.
• Document GitHub marketplace installation, secure authentication, project creation and editing, code export, updates, and troubleshooting.
• Add current desktop screenshots for the main Plugins page and Add marketplace flow, each rendered with an 8 px border radius.
• Add a compact GitHub repository card with hover and keyboard focus states.
• Cross-link the guide from the FlutterFlow CLI overview and the agent-independent build guide.
• Remove em dashes from the touched agent guide to match the requested documentation style.

Why

FlutterFlow's Codex plugin needs a focused onboarding path that distinguishes the CLI and desktop installation flows. The previous agent-independent guide did not cover plugin installation, secure credential handoff, or Codex-specific troubleshooting.

Impact

Readers can install the plugin from its GitHub marketplace, choose the correct Codex surface, authenticate without placing a token in chat, and follow the supported create, edit, and export workflows.

Validation

• npm run build
• git diff --check
• Local Docusaurus preview verified at /flutterflow-cli/codex

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[pinkeshmars]

This is great! LGTM