gpui: Document standalone app setup (#58766)

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>

crates/gpui/README.md

@@ -5,17 +5,48 @@ for Rust, designed to support a wide variety of applications.
 
 ## Getting Started
 
-GPUI is still in active development as we work on the Zed code editor, and is still pre-1.0. There will often be breaking changes between versions. You'll also need to use the latest version of stable Rust and be on macOS or Linux. Add the following to your `Cargo.toml`:
+GPUI is still in active development as we work on the Zed code editor, and is still pre-1.0. There will often be breaking changes between versions. You'll also need to use the latest version of stable Rust. Add `gpui`, and optionally `gpui_platform`, to your `Cargo.toml`:
 
 ```toml
 gpui = { version = "*" }
+gpui_platform = { version = "*", features = ["font-kit", "wayland", "x11"] }
 ```
 
+Everything in a standalone GPUI app starts with an `Application`. You can create one with `gpui_platform::application()`, which picks the windowing and text backends for the host OS, and kick off your application by passing a callback to `Application::run()`. Inside this callback, you can create a new window with `App::open_window()` and register your first root view.
+
+```rust,no_run
+use gpui::*;
+
+fn main() {
+    gpui_platform::application().run(|cx: &mut App| {
+        // ..
+    });
+}
+```
+
+### `gpui_platform`
+
+The features on `gpui_platform` are platform-specific, so the list above is a safe cross-platform default. If you build for a single platform, you can trim it:
+
+- **macOS** — Rendering uses Metal and is always available, but glyph rasterization needs `font-kit`. Without it, GPUI falls back to a placeholder text system that lays text out but renders no glyphs.
+
+    ```toml
+    gpui_platform = { version = "*", features = ["font-kit"] }
+    ```
+
+- **Linux / FreeBSD** — enable at least one windowing backend for desktop windows: `wayland`, `x11`, or both. These features also compile the renderer and text system, so no separate text feature is needed.
+
+    ```toml
+    gpui_platform = { version = "*", features = ["wayland", "x11"] }
+    ```
+
+- **Windows** — no features are required. Windowing uses Win32 and text uses DirectWrite. `font-kit` has no effect here.
+
+### Additional Topics
+
 - [Ownership and data flow](_ownership_and_data_flow)
 - [Accessibility](_accessibility)
 
-Everything in GPUI starts with an `Application`. You can create one with `Application::new()`, and kick off your application by passing a callback to `Application::run()`. Inside this callback, you can create a new window with `App::open_window()`, and register your first root view. See [gpui.rs](https://www.gpui.rs/) for a complete example.
-
 ### Dependencies
 
 GPUI has various system dependencies that it needs in order to work.