tools: fix anchors in generated documents

When an anchor tag is used within a pre tag, the link is not
distinguishable. This patch makes sure that the links are highlighted
by underlining them.

PR-URL: #2491
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api_assets/style.css

@@ -79,6 +79,15 @@ code a:hover {
   color: white !important;
 }
 
+.api_stability_0 a,
+.api_stability_1 a,
+.api_stability_2 a,
+.api_stability_3 a,
+.api_stability_4 a,
+.api_stability_5 a {
+  text-decoration: underline;
+}
+
 .api_stability_0 {
   background-color: #D60027;
 }