Backports redirects from #12528, adds indexing page

[C-STYR]

• backports redirects from Ports docs redirects from nginx-redirects.conf #12528
• adds indexing page for future sunsetting of older docs versions
• adds redirect for broken link in Teleport App

[ptgott]

I think we should forward port this page to master and backport it to branch/v8 and branch/v7 as well so users of all supported versions can view this page.

Also, just double-checking whether the intention is to make this page accessible only to visitors who know the URL, or if there will be a link from a layout in gravitational/docs.

[C-STYR]

Once I get the necessary approvals for this test page it will be included in master, but for now I don't want to have to update multiple PRs. It does not need to be backported - that will be handled during redirection.
Many roads and regex matches will lead here eventually, and there will be a link from the versions dropdown.