Summary:

- Document `gpui_platform::application()` as the entry point for
standalone GPUI apps.
- Add platform-specific `gpui_platform` feature guidance for macOS,
Linux/FreeBSD, and Windows.
- Use Git dependencies for the repository-based setup example.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

---------

Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>