Docs versions generating tons of 404s

By versioning our docs, considering that we only support a few versions and update them every month, we're generating a whole new site of 404s with every release. This is terrible for UX and SEO. We need to fix this ASAP.

Currently (2018-06-28) we have 808 pages returning 404s.

Options?

• blocking bots for every version: solves the SEO problem, but users will get 404s => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/288
• create a landing page for the versions (or use the archives page) to redirect all docs that aren't available online anymore (make versions available for download?) => https://gitlab.com/gitlab-com/gitlab-docs/issues/222 => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/289

Note that redirecting to another version will not make sense for versioning purposes

cc/ @lbanks @mikelewis @axil