Host the two latest major versions in the archives

Split from #5 (comment 1305700332).

We should host our last 2 major versions over on the archives instead of the primary site.

Couple of reasons:

• Logic for the version picker could be more simple (https://gitlab.com/gitlab-org/gitlab-docs-archives/-/blob/main/src/App.vue).
• Smaller footprint for the production site, which deploys more often.
• Simplifies explaining to people which versions go where 😄

Implementation

• Host last two major versions on Archives site (!24 - merged)
• Update links to previous major releases (gitlab-docs!3818 - merged)