doc: add 9.x to version picker and mark 8.x as LTS

PR-URL: #16672
Fixes: https://github.com/node/issues/16671
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/doc/html.js

@@ -189,7 +189,8 @@ function altDocs(filename) {
   }
 
   const versions = [
-    { num: '8.x' },
+    { num: '9.x' },
+    { num: '8.x', lts: true },
     { num: '7.x' },
     { num: '6.x', lts: true },
     { num: '5.x' },