Clean up references in doc comments in ui and theme crates (#3985)

This PR cleans up a number of references in doc comments in the `ui` and
`theme` crates so that `rustdoc` will link and display them correctly.

Release Notes:

- N/A

crates/ui/src/styles/typography.rs

@@ -38,7 +38,7 @@ impl UiTextSize {
     }
 }
 
-/// Sets the size of a [Headline] element
+/// The size of a [`Headline`] element
 #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, Default)]
 pub enum HeadlineSize {
     XSmall,