Tracking main-translation files needing to be pushed upstream

Background

GitLab needs visibility on what files have not yet been added to the main/master branches on upstream repositories. While @rasamhossain's Docs Translation Progress page provides valuable metrics, the specific files are not listed.

Solutions

1: An open MR comparing the differences

We can leave an MR open that compares the differences between main-translation and main/master. Since this is pointing to the fork, and fork's main/master branches are kept in sync with upstream, this provides a full list of files that differ. It automatically updates when main-translation is updated from a merge or a pull from upstream, or main/master is updated with a pull from upstream.

Right now the only open MR is GitLab, but we can open more if this proves to be useful:

• GitLab: https://gitlab.com/gitlab-com/localization/tech-docs-forked-projects/prod/gitlab/-/merge_requests/788