fix broken theme CSS links in the website (rust-lang#14379)

Currently, some parts of the
[website](https://rust-lang.github.io/rust-clippy/master/index.html)'s
CSS are not being applied. This seems to be due to a broken link to the
originally referenced CSS. I suspect
rust-lang/mdBook#1368.

changelog: none

Author: Alexendoo

util/gh-pages/index_template.html

@@ -19,10 +19,10 @@
     <link id="githubDarkHighlight" rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.6.0/styles/github-dark.min.css" disabled="true" /> {# #}
 
     <!-- The files are not copied over into the Clippy project since they use the MPL-2.0 License -->
-    <link rel="stylesheet" href="https://rust-lang.github.io/mdBook/css/variables.css"/> {# #}
-    <link id="styleHighlight" rel="stylesheet" href="https://rust-lang.github.io/mdBook/highlight.css"> {# #}
-    <link id="styleNight" rel="stylesheet" href="https://rust-lang.github.io/mdBook/tomorrow-night.css" disabled="true"> {# #}
-    <link id="styleAyu" rel="stylesheet" href="https://rust-lang.github.io/mdBook/ayu-highlight.css" disabled="true"> {# #}
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/rust-lang/[email protected]/src/theme/css/variables.css"/> {# #}
+    <link id="styleHighlight" rel="stylesheet" href="https://cdn.jsdelivr.net/gh/rust-lang/[email protected]/src/theme/highlight.css"> {# #}
+    <link id="styleNight" rel="stylesheet" href="https://cdn.jsdelivr.net/gh/rust-lang/[email protected]/src/theme/tomorrow-night.css" disabled="true"> {# #}
+    <link id="styleAyu" rel="stylesheet" href="https://cdn.jsdelivr.net/gh/rust-lang/[email protected]/src/theme/ayu-highlight.css" disabled="true"> {# #}
     <link rel="stylesheet" href="style.css"> {# #}
     <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.6.0/highlight.min.js" defer></script> {# #}
     <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.6.0/languages/rust.min.js" defer></script> {# #}