gpui: Update docs to reflect removal of View, ViewContext, WindowContext (#24008)

This PR updates function signatures, docstrings, and gpui's other
documentation to reflect it's new state following the merge of `Model`
and `View` into `Entity` as well as the removal of `WindowContext`.

Release Notes:

- N/A

crates/gpui/examples/set_menus.rs

@@ -37,7 +37,7 @@ fn main() {
 // Associate actions using the `actions!` macro (or `impl_actions!` macro)
 actions!(set_menus, [Quit]);
 
-// Define the quit function that is registered with the AppContext
+// Define the quit function that is registered with the App
 fn quit(_: &Quit, cx: &mut App) {
     println!("Gracefully quitting the application . . .");
     cx.quit();