Use Arial as fallback font instead of sans-serif.

On most platforms and browsers, `sans-serif` is equivalent to Arial.
However, on Firefox on Ubuntu (and possibly other Linuxes), `sans-serif`
is DejaVu Sans, a much wider font. This creates a larger shift in text
when the custom fonts finally load. Arial is a web-safe font, and
specifying it explicitly gives us more cross-platform consistency, as
well as reducing the layout shift that happens when fonts load.

src/librustdoc/html/static/rustdoc.css

@@ -136,7 +136,7 @@ h1, h2, h3, h4,
 #source-sidebar, #sidebar-toggle,
 /* This selector is for the items listed in the "all items" page. */
 #main > ul.docblock > li > a {
-	font-family: "Fira Sans", sans-serif;
+	font-family: "Fira Sans", Arial;
 }
 
 .content ul.crate a.crate {
@@ -482,7 +482,7 @@ h4 > code, h3 > code, .invisible > code {
 }
 #main > .since {
 	top: inherit;
-	font-family: "Fira Sans", sans-serif;
+	font-family: "Fira Sans", Arial;
 }
 
 .content table:not(.table-display) {
@@ -1301,7 +1301,7 @@ h4 > .notable-traits {
 
 .help-button {
 	right: 30px;
-	font-family: "Fira Sans",sans-serif;
+	font-family: "Fira Sans", Arial;
 	text-align: center;
 	font-size: 17px;
 }