Rollup merge of #74370 - Manishearth:re-spotlight, r=GuillaumeGomez

Reintroduce spotlight / "important traits" feature

(Reopened version of https://github.com/rust-lang/rust/pull/74111 because Github is broken, see discussion there)

Fixes https://github.com/rust-lang/rust/issues/73785

This PR reintroduces the "spotlight" ("important traits") feature.

A couple changes have been made:

As there were concerns about its visibility, it has been moved to be next to the return type, as opposed to being on the side.

It also no longer produces a modal, it shows the traits on hover, and it can be clicked on to pin the hover bubble.

![image](https://user-images.githubusercontent.com/1617736/86674555-a82d2600-bfad-11ea-9a4a-a1a9ffd66ae5.png)

![image](https://user-images.githubusercontent.com/1617736/86674533-a1061800-bfad-11ea-9e8a-c62ad86ed0d7.png)

It also works fine on mobile:

![image](https://user-images.githubusercontent.com/1617736/86674638-bda25000-bfad-11ea-8d8d-1798b608923e.png)

src/test/ui/feature-gates/feature-gate-doc_spotlight.rs

@@ -0,0 +1,4 @@
+#[doc(spotlight)] //~ ERROR: `#[doc(spotlight)]` is experimental
+trait SomeTrait {}
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-doc_spotlight.stderr

@@ -0,0 +1,12 @@
+error[E0658]: `#[doc(spotlight)]` is experimental
+  --> $DIR/feature-gate-doc_spotlight.rs:1:1
+   |
+LL | #[doc(spotlight)]
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #45040 <https://github.com/rust-lang/rust/issues/45040> for more information
+   = help: add `#![feature(doc_spotlight)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.