Preload the most commonly used fonts.

Don't preload italic font faces because they aren't used on all pages,
and when they are used, they are used sparingly so it's okay if they are
slower to load.

src/librustdoc/html/templates/page.html

@@ -7,6 +7,12 @@
     <meta name="description" content="{{page.description}}"> {#- -#}
     <meta name="keywords" content="{{page.keywords}}"> {#- -#}
     <title>{{page.title}}</title> {#- -#}
+    <link rel="preload" as="font" type="font/woff2" crossorigin href="{{static_root_path | safe}}SourceSerif4-Regular.ttf.woff2"> {#- -#}
+    <link rel="preload" as="font" type="font/woff2" crossorigin href="{{static_root_path | safe}}FiraSans-Regular.woff2"> {#- -#}
+    <link rel="preload" as="font" type="font/woff2" crossorigin href="{{static_root_path | safe}}FiraSans-Medium.woff2"> {#- -#}
+    <link rel="preload" as="font" type="font/woff2" crossorigin href="{{static_root_path | safe}}SourceCodePro-Regular.ttf.woff2"> {#- -#}
+    <link rel="preload" as="font" type="font/woff2" crossorigin href="{{static_root_path | safe}}SourceSerif4-Bold.ttf.woff2"> {#- -#}
+    <link rel="preload" as="font" type="font/woff2" crossorigin href="{{static_root_path | safe}}SourceCodePro-Semibold.ttf.woff2"> {#- -#}
     <link rel="stylesheet" type="text/css" {# -#}
           href="{{static_root_path | safe}}normalize{{page.resource_suffix}}.css"> {#- -#}
     <link rel="stylesheet" type="text/css" {# -#}