Enable rustdoc generate-link-to-definition feature on docs.rs (#5030)

You can see this feature in action
[here](https://docs.rs/sysinfo/latest/src/sysinfo/common/system.rs.html#46)
or on any of dtolnay's crates and many others. I found myself going
through your project code recently on docs.rs and I was a bit sad I
couldn't have this feature enabled. This should fix it at next release.
:)

crates/epaint/Cargo.toml

@@ -23,6 +23,7 @@ workspace = true
 
 [package.metadata.docs.rs]
 all-features = true
+rustdoc-args = ["--generate-link-to-definition"]
 
 [lib]