Add codex desktop app tutorial

[hwangjihun]

Summary

This PR adds a new tutorial for using the Codex desktop app in a CS2103T/AB3 workflow.

The tutorial covers:

• installing and signing in to Codex
• importing an existing project into the Codex desktop app
• selecting the correct Git repository root for AB3
• trying a small AB3-themed Codex task
• reviewing Codex-generated changes using the diff panel
• linked to official Codex app documentation for further exploration

New screenshots were added to illustrate the Codex setup flow, project import flow, and review workflow.

[damithc]

Thanks for the PR, @hwangjihun 👍
Some minor changes requested:

• Add yourself as the author of the initial version, at the bottom of the page (see https://se-education.org/guides/tutorials/intellijWindsurf.html for an example)
• As the website/app UI is evolving rapidly try to minimize the risk of the screenshots getting outdated soon. For example, we can avoid unnecessary screenshots and when we do use a screenshot, we can crop it to show only the relevant parts so that the surface area susceptible for outdating is smaller.
• Check for, and fix, minor indentation issues in the source file.

[hwangjihun]

Hi prof, thanks for the review. I have removed unnecessary screenshots and cropped the other screenshots so that only the relevant parts are shown. Fixed the minor indentation issues as well.