gpui: Fix intra rustdoc links (#37320)

The only warnings remaining are links to private modules/items, but I
lack knowledge to work out if the referenced modules/items should be
made public, or if the links should be rewritten into exposed
traits/items.

Links to associated items such as trait implementations have to be
written using full markdown format such as:

... [[ `App::update_global` ]](( BorrowAppContext::update_global ))

This is due to https://github.com/rust-lang/rust/issues/74563 which
sadly prohibits fully-qualified syntax:

... [[ `<App as BorrowAppContext>::update_global` ]]

Release Notes:

- N/A

Probably related to https://github.com/zed-industries/zed/pull/37072

crates/gpui/src/input.rs

@@ -72,7 +72,7 @@ pub trait EntityInputHandler: 'static + Sized {
     ) -> Option<usize>;
 }
 
-/// The canonical implementation of [`PlatformInputHandler`]. Call [`Window::handle_input`]
+/// The canonical implementation of [`crate::PlatformInputHandler`]. Call [`Window::handle_input`]
 /// with an instance during your element's paint.
 pub struct ElementInputHandler<V> {
     view: Entity<V>,