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/src/test.rs

@@ -101,7 +101,7 @@ impl<T: 'static> futures::Stream for Observation<T> {
     }
 }
 
-/// observe returns a stream of the change events from the given `View` or `Model`
+/// observe returns a stream of the change events from the given `Entity`
 pub fn observe<T: 'static>(entity: &Entity<T>, cx: &mut TestAppContext) -> Observation<()> {
     let (tx, rx) = smol::channel::unbounded();
     let _subscription = cx.update(|cx| {