doc: fix color contrast issue in light mode

Attributes are being highlighted as #f00 on a background of #f2f2f2.
That's a color contrast of 3.98:1, failing to meet the 4.5:1 requirement
of WCAG 2.1 AA. This changes the attribute color to #d00, which has a
color contrast of 5.09:1 meeting the 4.5:1 requirement.

PR-URL: #56272
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Claudio Wunder <cwunder@gnome.org>

Author: Trott

doc/api_assets/hljs.css

@@ -49,7 +49,7 @@
     color: #808080;
   }
   .hljs-attr {
-    color: #f00;
+    color: #d00;
   }
   .hljs-symbol,
   .hljs-bullet,