tools: add 12.x to alternative docs versions

PR-URL: https://github.com/nodejs/node/pull/27658
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>

tools/doc/html.js

@@ -394,6 +394,7 @@ 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' },