tools: add 12.x to alternative docs versions

PR-URL: #27658
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Сковорода Никита Андреевич <[email protected]>

Author: richardlau

tools/doc/html.js

@@ -384,6 +384,7 @@ function altDocs(filename, docCreated) {
   const [, docCreatedMajor, docCreatedMinor] = docCreated.map(Number);
   const host = 'https://nodejs.org';
   const versions = [
+    { num: '12.x' },
     { num: '11.x' },
     { num: '10.x', lts: true },
     { num: '9.x' },