Improve archives page

• Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/254
• Related to: gitlab-com&20 (closed) // https://gitlab.com/gitlab-com/gitlab-docs/issues/238
• Explains the default version, links to where the docs are in each repo
• Renames "current" to "latest released"
• Starts with the default version, descending all the way through to the offline archives

@axil can you please review/merge? Tnx!

cc/ @mikelewis