Create gray -> grey doc aliases (#5362)

As someone who uses "grey" instead of "gray", it is annoying that my
autocomplete can never find any of the "gray" color related things, so
this adds doc aliases for that.

* [x] I have followed the instructions in the PR template

crates/egui/src/style.rs

@@ -1015,6 +1015,7 @@ impl Visuals {
     }
 
     /// Returned a "grayed out" version of the given color.
+    #[doc(alias = "grey_out")]
     #[inline(always)]
     pub fn gray_out(&self, color: Color32) -> Color32 {
         crate::ecolor::tint_color_towards(color, self.fade_out_to_color())