Rollup merge of rust-lang#55760 - jonhoo:no-intermediate-fonts, r=GuillaumeGomez

Remove intermediate font specs

This is a (much) more constrained version of rust-lang#54772 that also aims at improving the situation in rust-lang#34681. It removes any font specifications that are not the "official" rustdoc font, and instead relies on the browser to provide the fallback font if the official on is not available. On Linux systems, this is particularly important, as fonts like Helvetica, Arial, and Times often look pretty bad since they're pulled from extracted MS fonts. A specification like `serif` or `sans-serif` lets the browser instead choose a good font.

Author: Mark-Simulacrum

src/librustdoc/html/static/rustdoc.css

@@ -70,7 +70,7 @@
 /* General structure and fonts */
 
 body {
-	font: 16px/1.4 "Source Serif Pro", Georgia, Times, "Times New Roman", serif;
+	font: 16px/1.4 "Source Serif Pro", serif;
 	margin: 0;
 	position: relative;
 	padding: 10px 15px 20px 15px;
@@ -114,7 +114,7 @@ h3.impl, h3.method, h3.type {
 h1, h2, h3, h4,
 .sidebar, a.source, .search-input, .content table :not(code)>a,
 .collapse-toggle, div.item-list .out-of-band {
-	font-family: "Fira Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
+	font-family: "Fira Sans", sans-serif;
 }
 
 ol, ul {
@@ -133,7 +133,7 @@ summary {
 }
 
 code, pre {
-	font-family: "Source Code Pro", Menlo, Monaco, Consolas, "DejaVu Sans Mono", Inconsolata, monospace;
+	font-family: "Source Code Pro", monospace;
 	white-space: pre-wrap;
 }
 .docblock code, .docblock-short code {
@@ -415,7 +415,7 @@ h4 > code, h3 > code, .invisible > code {
 }
 #main > .since {
 	top: inherit;
-	font-family: "Fira Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
+	font-family: "Fira Sans", sans-serif;
 }
 
 .content table:not(.table-display) {
@@ -1338,7 +1338,7 @@ h3.important {
 kbd {
 	display: inline-block;
 	padding: 3px 5px;
-	font: 15px "SFMono-Regular", Consolas, "Liberation Mono", Menlo, Courier, monospace;
+	font: 15px monospace;
 	line-height: 10px;
 	vertical-align: middle;
 	border: solid 1px;