fix missing github icon (#2120)

Author: ElliotFriend

src/css/custom.scss

@@ -192,15 +192,14 @@ select[data-testid="example-pairing-select"] {
 /* Header link icons */
 
 .header-github-link::before {
-  content: "\f09b";
-  font-family: "Font Awesome 6 Brands";
-  font-size: x-large;
-}
-
-.header-discord-link::before {
-  content: "\f392";
-  font-family: "Font Awesome 6 Brands";
-  font-size: x-large;
+  content: '';
+  width: 24px;
+  height: 24px;
+  display: flex;
+  background-color: var(--ifm-navbar-link-color);
+  mask-image: url("data:image/svg+xml,%3Csvg viewBox='0 0 24 24' xmlns='http://www.w3.org/2000/svg'%3E%3Cpath d='M12 .297c-6.63 0-12 5.373-12 12 0 5.303 3.438 9.8 8.205 11.385.6.113.82-.258.82-.577 0-.285-.01-1.04-.015-2.04-3.338.724-4.042-1.61-4.042-1.61C4.422 18.07 3.633 17.7 3.633 17.7c-1.087-.744.084-.729.084-.729 1.205.084 1.838 1.236 1.838 1.236 1.07 1.835 2.809 1.305 3.495.998.108-.776.417-1.305.76-1.605-2.665-.3-5.466-1.332-5.466-5.93 0-1.31.465-2.38 1.235-3.22-.135-.303-.54-1.523.105-3.176 0 0 1.005-.322 3.3 1.23.96-.267 1.98-.399 3-.405 1.02.006 2.04.138 3 .405 2.28-1.552 3.285-1.23 3.285-1.23.645 1.653.24 2.873.12 3.176.765.84 1.23 1.91 1.23 3.22 0 4.61-2.805 5.625-5.475 5.92.42.36.81 1.096.81 2.22 0 1.606-.015 2.896-.015 3.286 0 .315.21.69.825.57C20.565 22.092 24 17.592 24 12.297c0-6.627-5.373-12-12-12'/%3E%3C/svg%3E");
+  transition: background-color var(--ifm-transition-fast)
+    var(--ifm-transition-timing-default);
 }
 
 /* Hubble data dictionary formatting */
@@ -235,4 +234,4 @@ select[data-testid="example-pairing-select"] {
   font-size: 1em;
   line-height: 1;
   display: inline-block;
-}
+}