tools: add links to the stability index reference

This modifies the script that generates the docs
to create a static link from each Stability Index callout bar to
the Stability Index explanations in documentation.html.

PR-URL: nodejs/node#11664
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

tools/doc/html.js

@@ -322,9 +322,12 @@ function linkJsTypeDocs(text) {
 }
 
 function parseAPIHeader(text) {
+  const classNames = 'api_stability api_stability_$2';
+  const docsUrl = 'documentation.html#documentation_stability_index';
+
   text = text.replace(
     STABILITY_TEXT_REG_EXP,
-    '<pre class="api_stability api_stability_$2">$1 $2$3</pre>'
+    `<pre class="${classNames}"><a href="${docsUrl}">$1 $2</a>$3</pre>`
   );
   return text;
 }