Add /frontend to list of things to prefix with version

Closes #759 (closed).

With the recent frontend refactoring, some javascript is build under /frontend. For all the versions archives, we use a script that adds the versions prefixes, and we were missing the newly added frontend/ directory https://gitlab.com/gitlab-org/gitlab-docs/-/blob/master/scripts/normalize-links.sh.

For example, in the 12.10 version, you can see that all the other assets are prefixed with the version 12.10 , except default.js which is being picked from master, and that caused the ToC to break.

Luckily, this only happens when the versions are all present (in production), so we must to backport this up to 12.8 (although it wouldn't hurt to have this up to 12.4 as well). When you preview a single version as shown in https://docs.gitlab.com/archives/#offline-archives, this doesn't occur.