Remove intermediate font specs

This is a (much) more constrained version of #54772 that also aims at
improving the situation in #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: jonhoo

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;